3D Surface Metrology on PV Solar Wafers Karl- Heinz Strass cybertechnologies USA 962 Terra Bella Ave San Jose CA 95125 P: 408-689-8144 www.cybertechnologies.com
Introduction Solar photovoltaics is the fastest growing power-generation technology in the world, with an average growth rate of over 20 percent per year since 2002. Driven by advances in technology the conversion rate has steadily increased to well above 20 % in commercially available cells, while research labs have proven efficiencies of over 40%. The efficiency of a solar cells is the product of multiple parameters, one of which is reflectance efficiency, a significant portion of the (external) quantum efficiency, the percentage of photons converted to electric current. Surface topography, typically achieved by chemically texturing the wafer surface has a high degree of influence on the reflective properties of the wafer. Technology overview Traditionally, surface topography used to be measured with contacting stylus profilometers. However, due to the sensitivity of the surfaces, contacting method no longer meet the requirements of the industry. On the other hand, AFM technology, although it offers outstanding resolution, is not feasible due to the limited measurement area and scan speed. This lead to the advance of optical surface metrology systems, which can roughly be broken into two groups: Microscope based systems, with relatively high resolution but limited measurement size and scanning systems, where a point source is moved relatively to the sample surface and the surface variation is detected. The advantage of scanning systems lies within their capability to measure over large areas without impact on measurement resolution. They also provide much larger dynamic measurement ranges at high resolutions than microscope based systems. A major challenge for most optical systems is the measurement of the surface topography on samples coated with anti-reflective films. The films are designed to prevent the incident light from reflecting off the substrate, resulting in low levels of reflected light for the purpose of measuring the surface. cybertechnologies has developed a system that is optimized for these applications. cybertechnologies is a leading manufacturer of high-resolution 3D scanning surface metrology systems, with vertical resolution down to 3 nm and sample sizes of over 600 mm x 600 mm.
Objective A cybertechnologies cyberscan VANTAGE system with the small spot, ARC-optimized sensor is used to measure the surface of solar wafers. The surface parameters of interest are automatically determined. Measurement technology A laser beam is focused on the sample through an objective lens that moves up and down. The optical system is designed such that the maximum light intensity occurs when the sample surface lies within the focal plane of the objective. The reflected light is imaged through a pinhole onto a detector. As the objective moves closer or further away from the surface, the reflected light reaching the pinhole is defocused and the intensity decreases. The electronics collect a data point when the intensity of light imaged through the pinhole reaches a maximum. The sensor works on most any surface and is also widely used for measuring roughness and very small features on other engineered surfaces as well as on optically transparent or translucent films, like flux.
Measurement results A user- configurable screen layout shows all the pertinent information at one glance: - Measurement parameters selected, such as scan dimensions and lateral resolution - 3D raster screen, with false color controls - Selected 3D surface parameters - 2D line profile of user selected cross- section - 2D surface profile parameters - Abbott- Firestone curve and parametric results - Surface histogram with results and user defined thresholds
False color raster of scanned area on left and a 3D rendering of the surface below. The blue line indicates where the 2D line profile is extracted. The user can easily move the line up and down, or even change the angle of the virtual cut.
Advanced surface analysis Abbott- Firestone or bearing- ratio curve: Definition This analysis is useful to better understand the physical surface properties that the incident light interacts with as it more precisely describes the texture of a given surface. It essentially provides a fingerprint of the surface. The Abbott-Firestone Curve is determined mathematically by integrating the trace profile. With some mathematical modifications the concept can be adapted to a three- dimensional area. Image Source: Wikipedia, free encyclopedia File:Abbott-firestone curve.svg
The 2D line profile shows the raw surface signal, the extracted roughness and waviness profles superimposed or each of the individually. The software automatically employs the roughness filter norms according to DIN and ISO standards, independent of scan length. All selected surface metrology parameters are displayed together with the profile and can easily be exported into a text file or spreadsheet. The bearing-ratio, or Abbott-Firestone curve of the evaluated surface, with relevant parametric data.
The histogram shows the peak distribution of the evaluated surface. The blue band indicates user defined thresholds
Results Summary The PV solar cell surface evaluated has an average areal roughness Sa of 3.04 um, with an RMS roughness of 3.96 um. A slightly negative Skewness (Ssk) indicates that the surface has somewhat more deep and narrow valleys than sharp peaks. An Sku of 4.243, significantly above the 3.0 threshold points out that the surface has a strong flatness component with narrow peaks, while an Sku below 3 would have indicated a surface texture closer to a typical pyramid structure. Conclusion Surface finish quality is an important aspect of process control in the development and manufacturing of PV solar cells. Accurate and reliable metrology capabilities are essential in order to ensure consistent and high yielding product. cybertechnologies s cyberscan VANTAGE optical 3D surface metrology system provides unique capabilities for these applications, even on substrates coated with anti- reflective film. The advanced surface analysis software makes even the most challenging measurements fast and easy for operators, technicians or engineers alike. For more information on cybertechnologies s suite of 3D surface metrology systems please contact us at info@ cybertechnologies. com
Definitions Surface Parameter Definition Ra/Sa Average Roughness (2D/3D) Arithmetical mean of the profile height Rt/St Peak to Valley Roughness (2D/3D) Sum of the highest peak and the lowest valley within the roughness profile Rq/Sq Average Roughness (2D/3D) Root mean square of all heights in the roughness profile Rsk/SsK Skewness (2D/3D) Measure of the symmetry of the surface height. A primarily flat surface with many deep and sharp valleys in indicated by a negative Rsk/Ssk. A surface with many peaks protruding from a mostly planar surface is indicated by a positive Rsk/Ssk. It is important to ensure the sampling is identical in order to compare two surface measurement results with the Skewness parameter.
Surface Parameter Definition Rku/Sku Kurtosis (2D/3D) Measure of the uniformity of the height distribution. It is important to ensure the sampling is identical in order to compare two surface measurement results with the Kurtosis parameter. Wt Peak to Valley Height (Waviness) Sum of the highest peak and lowest valley within the waviness profile.
Other common applications on PV solar cells: