Automated Inspection Lowers Solar Cell Costs

The solar cell market is poised to experience exceptional growth, but continued success in the market will require manufacturers to drive production costs down. Automated optical inspection (AOI) using machine vision systems will play a key role in this cost reduction by speeding production and reducing waste. The challenge is to have the right vision system for each inspection task.

Unlike most high-tech markets, it is politics and not technology that is the driving force in the solar cell market. Rising energy costs, global warming concerns, declining fossil fuel reserves, and new governmental incentives are all contributing to a growing interest in solar technology. The industry has already seen a 35% compound annual growth rate (CSGR) since 1998 and is projected to remain at a 20% to 30% CAGR through 2011 (see Figure 1).

Figure 1 - Even conservative estimates predict double-digit growth in the solar cell market through the decade.

A key factor that could limit this growth, however, is the cost of photovoltaic energy. Sunlight is free, so the solar panels’ price will be the dominant factor in setting energy cost. Panel price must drop significantly in order for photovoltaic energy to be competitive with traditional alternatives (See Figure 2). Given the political climate, though, even without parity the lower the panel cost the faster the market will grow. With parity, the market could go as high as $31 billion by 2010.

Figure 2 - Solar panel prices must drop significantly for photovoltaics to become competitive on the large scale.

To reduce panel costs as well as meet growing demands, solar panel manufacturers will need to increase production efficiency. Automated optical inspection (AOI) can help in several ways. It is faster than manual inspection, allowing developers to speed their production process by removing the inspection bottleneck. AOI also offers higher reliability, precision, and accuracy than manual methods. This permits detection of errors earlier in the production process, reducing waste, and allows safe handling of thinner wafers while reducing production delays due to wafer breakage and subsequent cleanup. The vision systems performing the inspections can also provide immediate quantitative feedback on the location, type, and quantity of errors detected to support continuous process monitoring and improvement. The result of all these benefits is lowered production costs.

The Role of AOI

AOI can play a role in virtually every aspect of solar cell manufacturing, beginning with the silicon. Two different silicon solar cell types are currently in production: mono-crystalline and multi-crystalline. While these differ in formation and structure of the silicon ingot, they utilize essentially the same manufacturing steps to turn the ingot into wafers for cell manufacture (see Figure 3). This involves cutting the ingot into chamfered blocks and slicing the blocks into wafers. 

Figure 3 - Turning the silicon ingot into wafers is an early step in solar cell fabrication where AOI can help reduce costs.

Vision systems can automate both metrology and the quality inspection at this stage in the fabrication process. A three-dimensional system, for instance, can examine a cropped and chamfered ingot block for surface defects such as saw marks and chips as well as measure critical dimensions and angles on the edges (see Figure 4). This permits early identification of ingot sections that will yield flawed wafers as well as providing process control feedback. 

Figure 4 - Three dimensional AOI allows surface inspection as well as metrology on an ingot.

Once the ingot is sawed into wafers, AOI is able to examine the wafers before they enter the cell fabrication stage. Three-dimensional systems can look for bending and warpage that would create problems during photolithography and other processing steps that require a flat surface. Two-dimensional systems can measure wafer size and shape along with the homogeneity of surface texturing for both process control and quality assurance (QA). They can also inspect the wafer and surface for contamination and defects (see Figure 5). One such AOI system for silicon inspection – as well as glass, paper, film, metal, and other flat materials – comes from Dark Field Technologies. The company’s NxtGen™ system utilizes lasers and DALSA’s Piranha line-scan cameras to look for surface defects as small as 1 to 2 microns. Early identification of contamination lets manufacturers avoid later fabrication problems and poor yields by re-cleaning the wafer. Finding defects such as microcracking at this stage helps minimize waste by rejecting flawed wafers and saves testing cost by identifying early the wafer sections that will yield failing cells.

Figure 5 - Wafer inspection can identify and characterize defects such as micro-cracking before further processing, increasing process yields.

Silicon wafers become solar cells through an etching process that creates a textured surface followed by a semiconductor doping process that creates a diode structure (see Figure 6). A vacuum-deposited aluminum backing forms one electrical contact while the top contact is a printed metallic grid. A glass layer protects the remaining surface. This surface also receives an anti-reflective coating.

Figure 6 - The surface of a solar cell is textured and coated to help re-capture reflected light energy.

The texturing and coating serve to increase the cell’s efficiency in capturing optical energy. The anti-reflective coating uses interference effects to prevent reflected energy from escaping the cell. The texture helps direct whatever reflections do occur so that the light strikes the cell a second time, increasing the chances for absorption. Together they increase the cell’s efficiency at collecting incident light. 

The semiconductor fabrication processes that create solar cells offer several opportunities for AOI to provide support (see Figure 7). In addition to the wafer inspection already mentioned, AOI can be of great benefit in the measurement of texturization depth and uniformity and of anti-reflective coating thickness. Optical inspection of top-layer metallization can reveal a wide variety of defects, as well (See Figure 8).

Figure 7 - AOI can support solar cell fabrication at several key points, including incoming wafer inspection, texturization control, print inspection, and final test.

Figure 8 - AOI of the cell surface can quickly find errors in the metallization layer to weed out failures without costly testing.

Optical methods can also be applied to support the final test of finished solar cells. With the application of a reverse bias voltage, solar cells will emit light with a 1.15 um wavelength (see Figure 9). A camera capable of operating in the near infrared (NIR) region can capture this emission to provide a quick but detailed measurement of the cell’s operation over its entire surface area (EL test). The photocell material will also fluoresce in the NIR when exposed to photoexcitation, providing an optical additional inspection method (PL test). Both methods can reveal flaws not detectable by other means. 

Figure 9 - Injecting current into a solar cell will cause it to emit
infrared light that AOI can use to measure function and performance.

In most cases, manufacturing operations do not end with fabrication of the solar cell but go on to assemble cells into a panel (see Figure 10). At this stage machine vision systems have roles to play in both automated assembly and inspection. For assembly the vision systems can guide orientation and placement of cells in the frame. AOI can verify placement, inspect soldering, and evaluate the finished module for string quality, damage, and the like. 

Figure 10 - Solar cells assembled under glass into a sealed frame form a solar panel.

DALSA Addresses Multiple Needs

The many opportunities for AOI in solar cell manufacturing need to be met with a wide range of system options in order to obtain optimized performance at lowest cost. No one camera, for instance, can serve every inspection task. To find microcracks and to perform PL and EL testing the camera needs to be sensitive in the NIR spectrum. Surface metrology requires high resolution, while inspection of metallization needs only moderate resolution but highest speed. 

Further, different types of cameras better serve different tasks. The area-scan camera takes an entire two-dimensional picture at one time, much like a conventional snapshot. This approach best serves inspection operations where the object is stationary or accurate measurements of relative position on the surface are needed. Area-scan cameras are also the best basis for 3D imaging. 

A line-scan camera uses a different approach, exposing only one line of an image at a time. Object or camera movement then allows the build-up of lines over time to form the two-dimensional image. These cameras best serve inspection operations when the object is in motion or high resolution required over a large image area. A variant of the line scan camera combines the results of multiple sensors using time delay integration (TDI) to provide faster image capture at lower light levels.

Figure 11 - Line scan cameras with time delay integration (TDI)
help shorten exposure times by combining sensor results to
provide high resolution with fast image acquisition. 

While one type of camera cannot meet all AOI needs for solar cell manufacturing, one company can. DALSA offers a wide range of camera options for its vision systems so that customers can find the optimum cost/performance combination. For high resolution, DALSA’s 22M-pixel Pantera 22M cameras can resolve defects as small as 40 um.

Where speed and sensitivity are primary considerations, DALSA offers both area- and line-scan camera types with industry-leading performance. The Falcon 4M60 area-scan camera, for instance, uses patented CMSO sensor technology to provide 4M-pixel resolution images at 62 frames/second. The Piranha HS 4K640 TDI line-scan camera achieves data rates as high as 110 kHz, the fastest image capture speed in the industry.

DALSA is also capable of meeting the special needs of solar cell inspection with color cameras and cameras that work at NIR wavelengths. Color operation simplifies inspection of anti-reflective coatings because the reflected light spectrum provides an indication of coating thickness. Sensitivity in the NIR is essential for EL and PL testing, and NIR illumination is particularly effective in revealing microcracks. DALSA offers specialized InGaAs sensors for its cameras to provide NIR responsivity far superior to conventional CCD sensors.

The solar cell market faces an exceptional opportunity for growth, but production costs will set the pace and extent of that growth. Increasing production speed and yields, with corresponding reduction in cost, will require use of automated optical inspection methods at a variety of points in the fabrication process. DALSA offers a full range of cameras and systems to address the needs of each step along the way.

About the Author

Xing-Fei He, Senior Product Manager 
Xing-Fei manages DALSA’s Linescan and TDI product lines. In this role, he is responsible for all phases of product planning, development, marketing and sales, as well as execution throughout the product lifecycle. He has many years of experience in sensors and machine vision. Prior to this position, Xing-Fei served as Product Manager at JDS Uniphase (Milpitas, California) and Business Development Manager at LUXELL Technologies (Mississauga, Ontario). He holds a B.Sc. and M.Sc. Degree in Semiconductor Devices from Zhongshan University (Guangzhou, China) and a Ph.D. in Photonics from the Australian National University (Canberra, Australia). His areas of expertise include: imaging, displays and photonics.