Solar Cell Manufacturing meet Goals
How Near Infrared Line Scan Imaging helps
Image-processing in the near infrared (NIR) spectrum is well-established in the photovoltaic industry for demanding optical inspection procedures to check solar cells for micro-cracks, which can occur during crystal growth and wafer sawing. This powerful non-destructive analysis technique is used on production lines for quality assurance and to increase productivity. NIR technology is proving to be a reliable manufacturing tool for inline inspection and classification of a range of products beyond solar cell inspection as well, including checking printed circuit boards containing electronic parts and for large web and coating inspection.
Photovoltaic solar cells, fabricated from crystalline silicon wafers - which are typically 100-200 µm thick - can pick up defects at any stage of manufacture. Whenever these delicate wafers are handled mechanically the applied stresses may create micro-cracks that can grow unpredictably, leading to mechanical failure of the wafer during subsequent processing stages. It is important to remove wafers with critical micro-cracks early in the production process to prevent both wasted processing of already ruined wafers, and production-line stoppages caused by contamination from shards of wafers that shatter during processing.
Manufacturers of solar cells are constantly under pressure to find ways of increasing efficiency, improving quality and reducing costs. A properly designed automated optical inspection (AOI) system can help meet these goals. AOI systems are growing rapidly as the solar industry is reaching the stage of maturity in which machine vision undergoes mass adoption on the production floor.
There are numerous ways in which AOI aids in error detection and process control in a typical manufacturing environment. By using line-scan cameras, NIR is playing a major role. One approach is backlight and luminescence inspection at NIR wavelengths. Under backlighting conditions, a crack will scatter light and create a dark line against a light background that is readily detectable. Sensors need a resolution of 2-8 k pixels with 7-14-µm pixel size to be able to detect these defects. Another highly effective test technique uses electroluminescence or photoluminescence.
When excited, either electrically or optically, silicon luminesces in a band around 1050 nm in the NIR spectrum. High-resolution machine vision can detect micro-cracks, which appear as fine, dark lines in images resulting from excitation from a NIR source.
However, both techniques are not perfect. Sensor and camera selection are critical in developing an AOI system for backlit micro-crack inspection. The challenge is that charge-coupled device (CCD) image sensors normally have lower quantum efficiency (QE) at NIR wavelengths, resulting in a relatively weak signal. Moreover, camera systems vary considerably in their NIR sensitivity. Some cameras exhibit as much as 30-40% QE at 900 nm while others can be much lower.
Silicon NIR luminescence also can be so inefficient that the output intensity is quite weak. To generate useful images, cameras need to integrate using times that are often too long for in-line inspection at today's increasing production-line speeds. Sample wafers often have to be inspected off-line using stationary area-scan cameras.
To satisfy the photovoltaics industry's need for high-speed imaging systems, sensor developers and manufacturers constantly seek better alternatives. One approach is to use a sensor based on indium gallium arsenide (InGaAs) - which has greater sensitivity at 1.1 µm - instead of one that uses a silicon CCD. Unfortunately, this technology is also very expensive.
A technique called time delay and integration (TDI), combined with a high-resolution line-scan machine-vision camera, creates a system that has the potential to meet many of the challenges that solar-cell manufacturers are now facing. Line-scan technology enables images to be captured of wafers that are moving steadily along a conveyor at production-line speeds. Using a "multiple exposure" technique, TDI cameras effectively achieve higher responsivity and a better signal-to-noise ratio.
Teledyne DALSA cleverly combined key elements - enhanced QE in NIR and TDI technology - when it developed its Piranha HS NIR line-scan camera. Its responsivity is comparable to that of InGaAs-based systems, but it is more cost-effective. Teledyne DALSA's latest Piranha HS line-scan camera features improved sensitivity in NIR wavelengths, and benefits from TDI technology.
The Piranha HS NIR 8k camera is designed to meet the requirements of applications involving low-light conditions, such as checking solar cells, and printed circuit boards containing electronic components, but it is also targeted at large web inspection applications, and the food and pharmaceuticals industries. It effectively extends the company's High Sensitivity (HS) product line and provides a cost-effective system for imaging in near-infrared wavelengths from 700 to 1050 nm.
Improved sensitivity in the NIR region is critical to the success of equipment in a number of applications. The Piranha HS NIR's TDI technology enables multiple exposures for orders of magnitude increases in sensitivity while maintaining low noise performance - this is ideal for high-speed and low-noise applications. Camera systems based on NIR technology continue to evolve and are finding use in an expanding range of applications - from inspection of bank notes to pharmaceuticals to printed electronics. These industries have an opportunity to learn from the successful inspection process for solar cells.