The push to crank out solar cells more quickly brings problems as manufacturers work with ever-thinner silicon wafers.
Leland Teschler editor
Solar power still costs more than energy gained from burning coal or gas. That fact may put opponents of subsidies up in arms, but it’s great for companies that automate photovoltaic (PV) manufacturing lines. The reason: “Solar is driven by yield and throughput right now. They are trying to drive down the cost per solar cell so the technology approaches the economics of conventional energy,” explains Dan Tracy, senior director of industry research and statistics at SEMI, the overarching organization for the semiconductor industry.
The push to drive down costs and boost throughput is proving to be a boon for systems integrators and for automation companies. Overall spending on semiconductor- manufacturing equipment is down by double digits this year, but that’s not the case for companies supplying automation gear to the solar industry. There’s a lot to do.
“I would equate the PV industry today with the semiconductor industry in the early 1980s,” says Adept Technology Inc. Chief Technical Officer Dave Pap Rocki. About 75% of Adept’s Quattro robots sold so far have gone into PV applications, says Pap Rocki.
“A lot of the manufacturing processes in PV are just now being defined. It took SEMI to standardize the equipment in the semiconductor industry, but there has been no such effort in solar. So there are a lot of inconsistencies with shapes as, for example, with the interface to wafer transfer boats,” he says.
One factor that is complicating the task of handling solar-cell material is a trend toward thinner silicon wafers. Raw silicon has been expensive and in short supply. So there is an incentive to use as little as possible. This has led the industry to find ways of making substrates ever thinner, in some cases only about 100-microns thick. These wafers are delicate and notoriously difficult to handle without inducing damage in the form of chipping and cracking.
Moreover, state-of-the-art PV wafers are thinner than those for conventional ICs. They are manufactured in much higher volumes than are encountered in semiconductor chipmaking. So only a few lessons-learned about handling IC wafers apply to solar cells.
Handling problems begin when the wafers are sawed off the silicon ingot. “You get hundreds of wafers that all stick to each other because of the cutting oil,” explains Dr. Raul Fernandez, program manager of automation with the Texas Manufacturing Assistance Center, a unit of the University of Texas at Arlington. Fernandez was part of a group that helped define manufacturing and automation equipment for BP Solar, under a contract from the National Renewable Energy Lab.
It can be tricky to separate these wafers without breaking them. There are several ways of approaching the problem, but the devil is in the details. Many of the solar-cell and equipment makers that spoke with Machine Design for this article won’t discuss the subject because they have devised their own proprietary methods of wafer singulation.
Nevertheless, one general class of singulation method in use today employs air jets to pry the wafers apart and transfer them to cleaning and processing stations. Work done by Fernandez’s group for the NREL contract serves as an example. The researchers devised an air-levitation system that both separates wafers and moves them along a linear track, bidirectionally, without ever touching them. Called a valve-controlled bidirectional airlevitation track, the device proportionally pressurizes two plenums of oppositely oriented jets to generate horizontal motion. The individually controlled plenums connect to stationary jets aligned in opposite directions so the acceleration along the track is proportional to the horizontal sum of the two jets. A computer controls airflow into the plenums via two high-flow servovalves. The angled nature of the jets compensates for lift loss when a jet impinges on the edge of a wafer.
Fernandez says one advantage of the scheme is that it minimizes the stress concentrations induced during handling that can otherwise put cracks in the wafer. Cracking is more of a problem with the use of vacuum chucks, another technique sometimes employed for separating and moving wafers that are not superthin.
Bernoulli grippers are also widely used for handling and separating wafers. These devices operate on the Bernoulli principle, wherein airflow over the surface of the wafer generates a lift. One problem is that such grippers may have trouble plucking objects that are warped, as when picking up a thin wafer that sticks to the one behind it. In addition, Bernoulli grippers need some means of holding the wafer still so it doesn’t drift around as the gripper moves it.
All in all, “There are numerous handling issues yet to be solved,” says Adept’s Pap Rocki. “We are hearing a lot of complaints about wafer breakage, but they are not all due to the handling equipment. Sometimes it is because of the way the cells are stacked on top of each other and presented. When a mechanical device pulls the top wafer off a stack, it can damage the material beneath even though it has been singulated.”
Once wafers start moving through the production process, the emphasis is on transporting them smoothly with no jarring or shaking. “Otherwise, if you are using something like Bernoulli grippers, you could lose suction and drop the cells as you are moving them,” says Pap Rocki. “For the same reason, you must be able to stop precisely. Mushy stops increase the risk of bumping into nearby objects.” Overall, robotic equipment generally has no problem moving around wafers with accuracies of ±50 microns, say Adept officials.
The technology used for gripping wafers can affect the overall throughput of the manufacturing line simply because some can keep hold of wafers tight through higher rates of acceleration and deceleration. But the accel/decel rates are the least of the worries when moving a wafer with some sort of robot arm. “Solar cells are like a wing of an airplane when you move them through the air,” says Hai Chang, Adept Technology’s managing director of solar industries. “Wafers have different qualities depending on whether you move them edgeforward or corner-forward. How you move the wafer across a plane is important.”
Back-end blues The back end of the manufacturing process, where PV wafers are packaged into solar-cell modules, can also present handling problems. “Back-end operations require more dexterity and are more unstructured,” says TMAC’s Fernandez. “You are making connections and busing the cells together, in some cases perforating the backing sheets to make electrical contact with the cell strings. Like any other assembly issue, that can be challenging.”
One difficulty is in checking solar material for defects. Solar modules get probed for resistivity during manufacture. Cell makers sort modules based on their output, then charge a premium for the best products. But physical probing of thin wafers for these electrical measurements must be done carefully for fear of punching through the thin silicon substrate.
The handling that PV material undergoes in the manufacturing process increases the possibility that defects have been introduced somewhere along the way. PV manufacturers are using industrial vision systems to weed out these problems. It turns out that vision systems have a tough time spotting wafer cracks. Only a handful of industrial vision suppliers have come up with systems able to handle this task. Moreover, inspection can’t take place as PV material travels down an assembly line. Inspection must be under special lights that highlight the features of interest and employs pattern recognition software developed specifically for noticing PV defects. These capabilities are beyond what PV manufacturers can do themselves, so they generally are handled by vision specialty companies such as ICOS Vision Systems Corp. and Basler Vision Technologies that have developed inspection stations specifically for PV.
ICOS Vision Systems Product Manager Bruno Gouverneur says industrial vision systems frequently check for defects such as finger prints, cracks, impurities, warpage, saw grooves, and chipping. During cell inspection, vision checks the quality of the cell surface as well as that of the silver and aluminum layers on the backside.
Gouverneur says that vision suppliers consider several PV tasks to be challenging. These include the detection of microcracks, chipping, low-contrast defects, the thickness of coatings, and defects in logos. The problem isn’t necessarily in recognizing the defect, says Gouverneur, but in doing so quickly enough to keep up with production-line speeds. Currently most lines are operating at about 1 to 1.5 wafers or cells/sec, he says.
Unfortunately, there are some key differences between solar wafers and those used for integrated circuits that force vision suppliers to tweak their products specifically for solar lines. For example, explains Gouverneur, the vast majority of solar cells employ polycrystalline wafers whose crystalline structure is different for every wafer. So vision systems must be able to discriminate between ordinary crystal boundaries and defects. In addition, industrial vision systems for solar must use a field of view that is much wider than that for systems looking at ICs. So the cameras must have a higher resolution to handle a few of the more critical inspections.
One of the hot-bed areas for PV manufacturing development is South Korea. The Korean government wants the country to be the third-largest PV country in the world four years from now. So numerous well-known suppliers of semiconductor and flat-panel manufacturing equipment located there are working on PV gear derived from technologies for making ICs and displays. According to Adept Korea Co. President Jason Lee, equipment makers themselves handle most PV automation today because they are familiar with the processes involved. (Adept Korea is a private company independent of Adept Technology Inc.) But systems integrators are increasingly taking a role in automating PV lines as a means of cutting the backlog of solar-cell orders. One difficulty, Lee says, is that there are few system integrators familiar with PV processes, though the processes are simpler than those of IC making. Thus integrators climb a significant learning curve when getting into the field.
The situation with integrators is different in the U.S., according to Bosch Rexroth Sales and Marketing Manager Kevin Steele.
“In solar, it is less about cost right now than it is about throughput and performance and just delivering cells. There is a lot of work going out to systems integrators. The solar makers are trying to outsource as much as possible to move as quickly as possible,” he says.
Indications are that the dearth of equipment- interface standards for solar means there is a lot to outsource. “In all these jobs, you have to spend time analyzing specific needs and adapting to them because every line is unique,” he says.
Bosch has developed both manufacturing tools for solar as well as PV-automation equipment. Often, the two are intimately intertwined. That effectively means toolmakers as well as automation suppliers worry about moving around PV work in process. “Some of the tools must be custom made and incorporate handling mechanisms, which complicate matters when you work with superthin wafers,” says Steele.
Inside a Modern Wafer Handler
Conventional wafer-transfer systems use what are called comb pairs to lift wafers out of carriers and comb assemblies to retain the wafers. As wafers become thinner, this conventional method becomes problematic. The light weight of the wafers and their sharp edges make it difficult to consistently position the combs, which ultimately can cause wafer breakage. The combs are also optimized for a specific thickness. Companies that process multiple wafer thicknesses often need to exchange combs to handle the different thicknesses. Combs also experience significant wear from the sharp wafer edges, resulting in frequent replacements of this expensive consumable. One way of handling these difficulties comes from GL Automation, Dallas. The company uses a proprietary handling method that does not incorporate combs as the method of transfer. Its machines transfer wafers having thicknesses down to 90 microns without the need of exchanging combs.
Adept Technology Inc., adept.com
Bosch Rexroth, boschrexroth-us.com
GL Automation, glautomation.com
ICOS Vision Systems, icos.be/EN/01.shtml
Semiconductor Equipment and Materials International (SEMI), semi.org
Texas Manufacturing Assistance Center, tmac.org