Solar Nears Tipping Point

Solar cell manufacturing processes are benefiting from new motion control and automation technologies that are delivering higher efficiencies and sustainable quality.

"We have seen a huge investment in the development of solar products over the last five years," says Joe Campbell, vice president of marketing and sales for ABB Robotics. "A lot of these products are either coming into the pilot manufacturing stage or are starting to accelerate in volumes. More than ever, there is a lot of interest and activity in automating the whole process."

Campbell says cost is a bigger issue than it ever has been as the solar industry is starting to approach the tipping point where the cost comes down and solar becomes mainstream and economically viable, which he believes will put further pressure on the manufacturing process.

"What we see with solar cell manufacturing is that a lot of the processes are much better understood and well-characterized," says Campbell. "Typically what that means is that the pace of automation deployment will go up. Risk is out and costs are starting to come down."

Solar products, whether they are photovoltaic or concentrator technology, are requiring automated manufacturing processes. A second wave of automation is being applied to components such as frames, structure and the mechanical building blocks around the cell. Campbell says it is also interesting to watch the U.S. manufacturers who are very concerned about how their manufacturing footprint is going to extend globally. They are all thinking global footprint because they know that to be a major player they need to manufacture on at least three continents.

A key development for ABB is the introduction of its smallest-ever multipurpose robot, the IRB 120. The new six-axis robot has all the functionality and advanced design features of ABB's larger robots in an economical, lightweight model that will provide agility, accuracy and speed to a broad range of applications where a smaller footprint and profile are required. The robot weighs only 55 lb, so it can be easily mounted in a variety of configurations.

Campbell says the IRB 120 is a unique combination of a very small footprint and an efficient work envelope that is easily embedded inside other process machinery. Most of the applications for the 120 are embedded in other process equipment, wafer handling and placement equipment, or in a dedicated cell that is going to do interconnect placement and soldering. Designed with the solar photovoltaic (PV) cell and module manufacturing industry in mind, the IRB 120 is targeting applications such as string assembly, wafer handling and wafer sorting, providing more flexibility than Scara Robots often used for these functions.

Addressing Technology Imperfections

According to Joe Ottenhof, general manager for Beckhoff Canada, motion control and automation technologies are making a big difference in areas where solar cell production is still labor-intensive or where manufacturing now includes non-traditional components such as quality inspection.

He says many new manufacturing lines are incorporating 100 percent "in situ" quality inspections using nondestructive testing (NDT) and vision systems instead of doing a sampling product or offline inspection of wafers. That requires high-powered control systems because typically the control system has been involved with the handling and manufacturing of the cell itself, and other more data-intensive tasks were handled offline.

"The whole trend to doing operations 'in situ' is to address the fact that this is not a perfect technology. By being able to do the quality online 100 percent inspections, manufacturers have an opportunity to 'grade' the product according to its quality level. The quality level of the product determines its selling price," says Ottenhof.

One solar cell manufacturer does "virtual tracking" through its motion control system on every wafer produced. Hundreds of thousands of wafers a day are tracked using the encoder counts from the servomotors in the system. So if at any manufacturing point a test is done on a wafer, the data system knows that the wafer at position X in carriage 23, silicon wafer 4, has a certain quality level.

The goal is to collect data on every wafer at virtually hundreds of positions through the manufacturing process, and create a record of all its quality characteristics. Cells are degraded or rejected if they don't pass certain quality parameters. But also from a tracking perspective, it is a manufacturing differentiator to be able to go back and analyze issues that may develop after field testing and installation. Being able to go back not only to a panel but to an actual solar cell within an installed panel allows solar cell manufacturers to know with a great degree of confidence that they have a well-defined process.

Robotics and Vision Advance

"One very important and established trend in solar cell manufacturing is companies moving from manual labor to automated technologies," says Hai Chang, director, Solar Industry Business for Adept Technology Inc.

Chang says that Asian manufacturers in particular are now recognizing the importance of automation and following the best practices of Western manufacturers as a way to lower overall costs by reducing wafer breakage and increasing yields. Chang says Adept's Quattro s650H robot is extremely fast, but you are never going to be able to handle solar cells as fast as you can handle chocolate, food or other smaller, less delicate items. Even so, breakage rates at customer sites have been reported to be less than 0.03 percent, compared to breakage rates of 1 to 3 percent with manual handling

That may be one reason why one trend in robotics technology is smoother handling. Precise control over acceleration/deceleration rates and the rate of change of those accelerations and the use of S-curve algorithms allow users to fully optimize all motions for maximum speed and smoothness while carrying the delicate wafers.

Todd Reynolds, an applications engineer for Adept, says that when you are carrying a wafer, it essentially acts like a very small, delicate wing. The goal is to minimize the stress and deflection of the wafer while maintaining speed. Optimization requires a trade-off of these two variables.

Reynolds says that vision processing for solar cell applications is also continuing to advance and is increasingly being used with robotic handling to quickly locate and orient solar cells before placing them into loading trays or nests. When trays and nests are loaded into PECVD equipment, for example, there is often less than 1 mm of spacing between the product and the edges of the nest they are placed into. Without the use of integrated vision-guidance technology, wafers can be damaged as they are placed.

As vision technology continues to advance and faster algorithms are developed, Reynolds says he believes the advances can boost throughput, and applications incorporate more numerous and increasingly complex inspections.

"With a single image we can define the wafer's location and orientation and run some simple inspections such as checking for edge/corner chips, cracks, breaks and physical dimensions before handling," he says.

Modern vision inspection technology also creates greater consistency compared to manual inspection elsewhere in the line and helps eliminate the subjective judgment of human operators while performing quality inspections. Hue inspection of anti-reflective coatings on silicon cells is an example of an operation that is still commonly performed using manual inspection with a chart that includes a range of acceptable hues. It is a perfect example of where machine vision delivers a superior solution.

Better Feature Geometry, Repeatability

According to John Lindell, product manager for Aerotech's photovoltaics product line, the trends in solar cell manufacturing are continuous improvement in application of materials, better feature geometry and better overall process control. Instead of a "throw it away" mentality, the focus is finer and straighter features over the length of the panel and how to squeak out that last little bit of power.

An example of improving throughput is the SolarScribe, a device for selectively and precisely removing material from solar panels that have had individual layers applied. With the move to thin film coatings applied on photovoltaics, each layer is laid down and scribed.

Lindell says an important technology that boosts throughput is referred to as a split axis system. One axis down below moves the panel back and forth, and a separate axis mounted to a bridge up above carries multiple laser scribe heads. As the panel passes underneath, the lasers turn on and scribe those materials.

"The split axis arrangement lowers the moving mass of the scan axis, which means we can go faster," says Lindell. Typically incorporating four or more scribe heads, the split axis configuration provides much higher throughput because the number of passes the panel makes to create all of the required scribe lines is much less. The reduced number of motions per panel also helps increase the life and lower manufacturing costs.

Beyond lowering the moving mass, the split axis arrangement produces scribe lines which are now only dependent on the dynamic straightness of the bottom axis. With a traditional stacked XY system, the load is offset, and accelerating and decelerating affects straightness. But by splitting the two axes, operation is very repeatable and produces lines that are straight and parallel.

Another significant technology in these systems is direct drive solutions using linear motors. Applications for scribing devices are often accelerating at 5g's and moving at 2.5 m/sec in plants that operate 24/7 which puts high requirements on the motion system. Lindell says the use of the linear motors is important to get the speed and trouble-free long life desired when systems are moving masses which can be on the order of 50 kg. Another key advantage is that the linear motors can be stacked together in parallel where other solutions would require a bigger, heavier, higher horsepower motor.

Because linear motors are direct drive and non-contact, and typically the encoders used are also non-contact, the only mechanical contacts are the mechanical bearing rails. The result is very good dynamic straightness and dynamic straightness repeatability.

Related Webcast: Motion Control/Automation Solutions for Solar Cell Manufacturing

In this Design News webcast, design engineers will gain insight into how manufacturers in one of the hottest and fastest-growing technology areas today are leveraging the latest precision motion control technologies, robotics, networked automation inspection stations and automation solutions to make more units at a lower cost. This webcast will be available at www.designnews.com starting Jan. 25.