Machine Vision Solves Solar Problems Across the Spectrum

Solar cell manufacturing has been a bright spot for the machine vision industry during recent years. Not just because of increased demand and investment, but also because of the nature of solar cell manufacturing, driven by their various designs, and the many process steps that can benefit from machine vision quality control systems. The latest developments in solar-centric machine vision can be found across the spectrum – the electromagnetic spectrum, that is. Whether its monocrystaline, polycrystalline, or thin-film photovoltaic (PV) panels, machine vision is helping manufacturers to boost yields and set the stage for a renewable energy revolution.

A Bright Spot

Energy consumption for today’s modern world is growing by approximately 10% every 5 years while fossil fuel reserves are finite. In an effort to match supply with demand, government has increased financing for photovoltaic development by 400%, while private funding tripled in 2007, and then tripled again in 2008, according to Peter Klaerner, Staff Process Engineer at SolFocus Inc., and keynote presenter at The Vision Show in 2009. The AIA’s Director of Market Analysis, Paul Kellett, estimates that the solar cell market will grow from $13 billion in 2009 to $30 billion by 2012 (see this article).

As Kellet points out in his article, the solar cell market is similar in many ways to the semiconductor inspection market, especially as it relates to silicon wafer based photovoltaics (PV). As cheaper thin-film PVs have grown in efficiency, machine vision companies can also pull from their expertise serving the LCD flat panel industry because thin-film solar cells are in many ways similar to an LCD glass with applied anti-reflective and color filters. The larger size of thin-film PVs compared to silicon-based PV cells, along with their easier, cheaper manufacturing processes, are driving money into thin-film PV development and manufacturing.

Basler Vision Technologies (Ahrensburg, Germany) is one company using its LCD panel expertise to help the thin-film PV manufacturing market. “One of the most serious problems our customers are facing is glass breakage during the coating process caused by defects in the glass that are propagating during thermal and mechanical stress,” explains Enzio Schneider, Basler Sales Engineer and PV Specialist. “We’ve built a system that can do 100% inspection of the glass as it comes into the manufacturing line. At this point the glass may have a Transparent Conductive Oxide (TCO) coating, which we can inspect for voids and other defects. At the same time, we inspect the edge of the glass, which is usually made using mechanical grinding. If there are any microcracks or edge chips along the edge caused by the grinding process, the glass is more likely to break during subsequent manufacturing steps. By implementing our system, one manufacturer was able to avoid 95% of glass breaks. Usually, a glass would break once a month, and cause up to 8 hours in downtime. The return on investment of our system in this case was less than two months.”

Looking Beyond the Visible

Microcracks are too small to see with the unaided human eye, but they’re not the only hidden defect when it comes to solar cell manufacturing. Sinfrared, a XenICs (Leuven, Blegium) company, sells short-wave infrared (SWIR) cameras to the solar manufacturing industry to check a variety of manufacturing steps, from the TCO layer to silicon wafers.

Thin-film PVs essentially work like a reverse LED, using the electron band gaps inherent to different semiconductor materials to convert light (photons) into electrons, rather than the other way around. Therefore, applying an electric charge can cause a thin-film PV material to emit faint light. The color of the light depends on what layers have been applied to the glass. When it comes to TCO and several of the metal oxide layers, the emitted light is in the SWIR part of the spectrum. This phenomenon can help PV manufacturers to check the quality of the ‘invisible’ coating prior by applying an electric field, taking a SWIR image of the glass, and looking for intensity variations that indicate gaps, thin spots, or corrosion of a specific layer.

Infrared (IR) is also an important spectral band for silicon-based solar cells. Silicon wafers, including monocrystalline and polycrystalline wafers, are relatively transparent to IR light. Silicon carbide is not. Sometimes, when growing a silicon crystal, the crystal can include silicon carbide defects, which can break the sawing wires used to slice a silicon boule into individual wafers, resulting in downtime for the production line. Sinfrared’s cameras are also used to inspect silicon wafers to look for silicon carbide or other material defects inside silicon wafers.

Recognizing the need for multispectral, high-resolution solutions for the solar manufacturing industry, machine vision camera specialist, JAI (Glostrup, Denmark) has developed the C3 Advanced line of multi-CCD cameras in addition to a variety of monochrome cameras. Unlike traditional machine vision cameras that use a Bayer filter applied to the CCD, which reduces spatial resolution by approximately a third, JAI’s color cameras use a prism to split the incoming light into its RGB components and capture each light stream with a separate CCD. This eliminates the need to interpolate between different colored pixels, maintaining the high spatial resolution while delivering a quality color image.

“Our OEM customers want to use vision systems to both position the wafer in a wide area of interest, and then be able to zoom into a specific area,” explains Tue Moerck, Director of Marketing for JAI. “When they zoom in, they want a very high modulation transfer function (MTF) so they can see the smallest details, which may only measure a few tens of microns. In that case, we use all three CCDs to capture the wide area image, and then switch to the blue CCD with blue light for the zoom image because it is the shortest wavelength of the visible spectrum and will yield the best image for small details.”

JAI also offers 4-CCD line scan cameras that can include a near infrared (CCD) optimized sensor for detecting gaps or defects in special coating layers, or cameras with UV-optimized CCDs that either have an additional collagen layer that converts UV light into visible light, or are built without microlenses and glass, which absorb UV light.

“There’s quite an interest in UV cameras because they can see smaller features than visible light cameras,” continued JAI’s Moerck.

OEMs for wafer and solar inspection equipment are also exploring the possibility of going to multi-tap, multi-channel cameras to reduce the number of cameras needed to check large area thin-film solar cells or large silicon wafers. “Few camera makers can develop the right algorithms to balance data from different channels, and/or cameras so that you don’t mistake a change in output for a defect. Today, most OEMs developing inspection equipment for these industries will go with four cameras rather than one larger array camera with multitap capabilities. However, multitap is the way the industry is going to keep speed and resolution high.”