Optical Inspection: Quality is an Absolute Must

Production capacities in the manufacturing of photovoltaic products have increased significantly over the last few years. By the end of 2011, more than 40 GW will have been installed and production will therefore markedly exceed current demand. As a result, manufacturers in this sector will face new challenges. In industrial production of photovoltaic products, optical inspection systems ensure that a product’s quality level remains consistently high at high throughputs.

The market has come to expect that products will continue to improve, offering higher performance and better quality, while at the same time remaining cost effective. Solar cells are being produced with finer and finer structures. As a result, there is an ever-increasing demand for even greater precision during production. When it comes to customer demands, new technologies such as double printing or selective emitters are setting new standards.

A major contributory factor to a company’s success is the quality of its products. In addition to measurement of electrical performance, manufacturers are increasingly turning to optical inspection systems as a means to achieve required quality levels. These systems not only categorize the individually inspected products by quality, but they also supply valuable data on how to control and optimize the overall manufacturing process. The manufacturer therefore has the opportunity to benefit from the optimization and cost reduction potential offered.

The product lines Solarscan, Powerscan, Scribing Inspection, Patternscan Ribbon, and Formscan Solar represent a comprehensive portfolio of optical inspection systems. These systems are being implemented throughout solar cell manufacturing, during the production of modules and for thin-film production on glass substrates. Optical inspection systems are also being utilized for incoming inspection of structured cover glass as well as for 3-D-inspections of parabolic solar reflectors. Each manufacturing sector has its own specific requirements and the available systems can be customized to meet them.

 

The finishing process – From wafer to solar cell

In the cell manufacturing process, silicon wafers are put through a series of processing steps before they become a finished solar cell. In a first step, the cut wafers are cleaned. After that, they are treated in chemical baths to remove any damage from the cutting process; this also produces the desired rough surface structure that increases their luminous efficacy. The electrical field in a solar cell is built up in a p-n-junction. In most cases, this is created by processing the cells in a furnace in a phosphorus environment. During the process, a phosphorus glass layer is formed on the surface of the cell, which is then removed in a subsequent etching step. In addition to the rough surface structure of the wafer, the cell’s anti-reflection coating (recognizable by the typical blue color) also increases the luminous efficacy. This coating is usually applied using PECVD (plasma-enhanced chemical vapor deposition).

 

Inspecting the results and avoiding additional costs

The next step is to print the necessary structures on the cells. Their purpose is to pick up the current that is generated and to allow the cells to be joined up to one another by soldering. After subsequent sintering in a furnace at approx. 900 °C, the cells are classified and sorted. The process chain is quite extensive and can lead to a multitude of different processing deviations or result in defects. Inspection systems that have a resolution ranging between 1 and 84 megapixels are applied in the optimization process.

When wafers are received, optical and electrical systems measure the geometry of the wafers. They seek out any surface defects, such as inclusions or saw marks. They also test the wafers for µ-cracks, to determine the life cycle of the load carrier as well as to measure the thickness and the deflection of the wafers. The goal in this case is to identify all wafers that are already damaged so that pointless work, tying up equipment and consuming valuable time, as well as additional material costs can be avoided.

 

Monitoring process accuracy

In cell manufacturing, cameras are used at different positions along the processing chain to check on cell alignment and/or breakage. After the wafers have been textured (thus creating a rough surface), systems inspect the homogeneity and the roughness of the surface structure. These systems supply the information needed for optimization during the wet chemical process. In addition, the user is able to categorize the wafers on the basis of visible inclusions in the silicon.

Wafers with an AR (anti-reflection) coating are assessed on the basis of their homogeneity, their color impression, and their AR coating layer thickness. Before the wafers are printed, the systems optimize the PECVD processing step and sort out any defective wafers. This allows the user to maintain an equally high level of quality during manufacturing and to avoid any added costs.

Immediately after the separate screen printing steps, i.e. front print and rear print, optical systems test and measure alignment of the print, determine any print errors, and supply information about changes to the screen, i.e. build-up or damage. By using these systems, the user is able to react immediately to any problems in the printing process. In the simplest of cases, the screen can be cleaned within seconds and this will allow the quality of the subsequent cells to be optimized. Tolerances for the alignment of the print surface vary. Depending on the technology used, it is typically between ±200 and ±20 µm. In order to be able to monitor the process accuracy, automated optical inspection systems are critical. Fine structures (“fingers”) are printed on the front side of the cells. Currently, they have a width of between 70 and 130 µm. New technologies are being developed in an attempt to reduce the width of these structures to 30 to 70 µm. Depending on the width of the structures, the optical inspection systems must be designed with sufficient resolution. Typical defects that are found during the inspection of print results are: interruptions, tapering and bulging of the fine line structures, deviations in the structure widths, holes in the print surface, paste spots between the print patterns, and cell damage. The rear side of the cells is also inspected, by checking the print alignment and determining any missing material on the print surface.

 

Classification of cells

The final step in cell manufacturing involves the classification of the cells. A critical aspect in this process is determination of the cell’s efficiency factor. The output of the cell is also becoming increasingly important in evaluating cells and detecting any previous damage. Such evaluation includes, among other things, searching for µ-cracks and the overall visual impression of the cell. If the customer has the choice, he will more than likely choose the cells that are technically and visually superior if the prices are comparable. When classifying solar cells, several different inspection systems are used. Optical inspection systems check the printed surface of cells (front print and rear print) and assess the color impression (AR coating). The print inspection evaluates the print alignment. In addition, the systems also check for errors on the print surface such as interruptions, tapering, or bulging in the fine line structures. During the print inspection, however, the systems also inspect for deviations in the structure widths, holes in the print surface, paste spots between the print patterns, and damaged cells.

In addition to evaluating the print alignment, the calculated values are used to bond the cells in the flasher (sun simulator). Even the slightest deviation during the cell bonding process can lead to false measured output or even cell breakage when the solar cells are assessed in the flasher. This makes it necessary to align the cells either before or during bonding. However, use of a mechanical process employing “stops and guides” to bond the cells may result in even greater damage. This is why manufacturers are relying increasingly on optical solutions to align the cells.

Evaluation of color impression concerns the color impression of the entire cell, the homogeneity of the coating, and any local color spots. A particularly difficult aspect of this process is how to evaluate multicrystalline cells and to detect “direction specific” color defects. The key feature of flasher (sun simulator) classification is calculation of the cell’s characteristic curve and hence definition of the cell’s power output; additional electrical measurements are also performed.

 

Visualizing the invisible

Two other inspection systems supply additional information that remains invisible to the human eye and to any systems that were hitherto available. Electroluminescence exploits the inverse photoelectric effect. A cell that is energized in the direction of flow (forward bias) generates a weak self-luminosity (luminescence). This is detected using special cameras that reveal important information about µ-cracks, interruptions in the print surface, and cell homogeneity. Hot spot inspection energizes the cells in the opposite direction (reverse bias). Ideally, a cell should be blocked in this direction and as a result not allow current to flow. In reality, though, this can cause short-circuits (so call shunts, shunts may cause also called hot spots) which can lead to numerous problems. Even though the flasher is able to detect short-circuits, it cannot determine whether the cells heat up as a result (i.e. whether they are dangerous or not). Here too, cells are viewed with special cameras that classify these hot spots. Such hot spots can cause serious damage to the module at a later juncture.

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EUPVSEC 2011 Hall A4 — A4