Anything but stress

Kevin Reddig likes to draw a comparison to explain the complexity and technical challenges of wet wafer singulation: “Think of it as a wet book whose pages you have to separate from each other without damaging them.” Not an easy task without ripping the pages. But the comparison is not quite accurate, a fact confirmed by employees specialized in photovoltaics automation at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, Germany.
Raw wafers, currently 160 to 180 micrometers thick, are thicker than a page of a normal book and – unlike wet paper – are not at all flexible. Also, the pages of a book are not freshly sawn and sticky with saw residue.
Wafers are created by using wire saws to slice a silicon ingot this is glued to a glass plate, which is not cut in the process. Afterwards, the raw wafers hang from the glass plate like the teeth of a comb. Between the teeth is a residue of sawdust, consisting of glycol, silicon carbide, and metal fines. The abrasive saw residue, called slurry, is ideally washed off after sawing, but such systems are far from standard equipment in many production facilities. But because the spaces in between are often as thin as the wafers themselves, cleaning is difficult.

Limits to manual labor

Even in the best case, some slurry residue mixed with water and chemicals remains after the washing. This mixture not only creates comparatively dirty working conditions, it also makes the wafers difficult to separate without damage. Only skilled workers can do it by hand, without high breakage rates. For wafers with 125 millimeter long sides, the average rate is a few percent, which explains why raw wafer singulation is still very often done by hand.
But as wafers get thinner and larger, even careful workers reach their limits. “The 125 millimeter wafers are not a problem,” Reddig says. “But at 156 millimeters you need both hands.” That affects the workers’ sense of touch and coordination; not ideal circumstances for maintaining quality work at ever increasing production rates. The result is higher breakage rates and reject material costs. The expense is unsustainable under the current cost pressure in photovoltaics, which is why wafer manufacturers worldwide now feel compelled to close the existing gap in automated production – especially in new lines.
Thus, it comes as no surprise to Christoph Jansen, an employee of Bavarian mechanical engineering company AMB of the Meyer Burger Group, that the market’s interest in singulation systems is on the rise. “This is a very hot topic. More so because there aren’t many systems on the market.” Singulation system manufacturers are working around the clock to change that – such as the five German companies considered the global leaders in the segment: ACI-ecotec, AMB Automation, Decker-Anlagenbau, Gebrüder Schmid and Rena.

Re-thinking automation

At first glance, it is hard to see why it has taken so long for automation companies to score success with their wafer-separation solutions. One reason is that they have had to re-think their approach in order to deal with conditions such as abrasive particles in a wet environment, thin and brittle silicon slices, and high adhesion forces. “There were projects aimed at reproducing hand sorting as accurately as possible,” Reddig recalls.
The result was complex machines full of sensors and actuators. They proved to be not only vulnerable to malfunctions, but also difficult to repair from the outside and complicated to maintain. Potential buyers were not interested, generally ignoring the automated alternative. To arouse interest, engineers had to rise to a new kind of challenge; instead of building technically complex machines, they had to simplify. Less was often more.
“With our machine, we’ve learned from experience what to pay attention to,” says Felix Jäger, who is responsible for photovoltaics at the manufacturing systems engineering firm ACI-ecotec in Germany’s Sankt-Georgen. “Because the environment is so aggressive, we consciously avoid exposed machinery and water-sensitive parts in the work area, and we use electronic components as sparingly as possible.” The result of the learning process looks relatively simple. But the machine works the way customers want it to, which is why the equipment built by this engineering company in the Black Forest has been one of the few systems in automated production lines since as early as 2006 in such companies as Wacker-Schott, Deutsche Solar and China’s LDK.
Watchwords like “robust and dependable” and “simple and easy to understand” were the automation company’s guiding design principles for its wafer singulation systems. For Schmid Group PV wafer product manager Reinhard Huber, there is no question: “Regardless of the complexity of the system details, the entire wafer conveyance mechanism and any parts of the machine that come into contact with water have to be easily accessible.” That is important, he says, because machine operators help decide which singulator to buy. “It’s definitely not misguided to consider the wishes of operators and develop machines that are easy to use.” Wafer manufacturers have other concerns as well. They expect three things from automated singulation: a lower breakage rate, higher throughput, and a resulting drop in production costs. To get an idea of how much costs can be brought down, consider this example: at a wafer price of five dollars and an hourly throughput of 2,000 wafers, 100 dollars (about 70 euros) can be saved for each percent of breakage reduction. Every percentage point counts – especially at high production volumes. Higher hourly throughput also positively affects cost – assuming healthy demand.
That equipment manufacturers are catering to these needs is evident not least in performance data, which are generally comparable: breakage rates well under one percent, and a maximum throughput of 3,600 wafers per hour. Christoph Jansen is convinced that manufacturers unable to keep pace with those specs have no chance on the market, “If the singulation system is a bottleneck in the line, no customer will buy it.”
However, one thing is worth bearing in mind with regard to breakage rates and throughput: performance specifications achievable in practice depend considerably on upstream and downstream process steps. For instance, breakage rates for singulation listed on specification sheets have to be thought of as subject to certain conditions. “Breakage rates increase for pre-damaged wafers. And that goes for any singulation machine, regardless of how carefully it handles wafers,” says Kay Rehberg, one of three executives of Decker-Anlagenbau in Berching, Germany, near Nuremberg.
The sawing and pre-washing process parameters set by wafer manufacturers can either increase or decrease damage to wafers prior to singulation. So far, however, no process exists to reliably determine wafer quality prior to singulation. Rehberg has called on the research institutes to step up their efforts in this field. “It would be very helpful if we knew more about this. Then we could further optimize the process.”

Other bottlenecks

Processes following singulation – fine washing and qualification – also affect throughput. It does not make much sense to singulate any faster than those two stations can work. “One wafer per second is currently the limit,” says Felix Jäger. That is how fast the inspection system at some 90 percent of manufacturers qualifies and sorts wafers at the end of the production line. The system is manufactured by Henneke Systems, another member of the Meyer Burger Group.
But the quality of the pre-wash also significantly limits throughput. Christian Nitz, an employee of equipment manufacturer Rena in the Black Forest town of Gütenbach, points to the decisive factor: complex loads known simply as “stress” in the industry. “Only good pre-washing can drastically reduce stress on the wafers during singulation.” Reiner Huber is convinced that good pre-washing can also simplify the entire wafer production line and speed up the process as a whole. “So far, no one has managed to singulate more than 3,200 wafers per hour on a single lane.” The limitation on throughput has been due to previous pre-washing methods, which take time and leave sawing residue between the wafers. “Manufacturers have had to limit line capacity to prevent processing stress, which is why we divide the wafers up among two or three lines.”

Processes vary

For the core functions of these machines – stress-free singulation and secure conveyance – all of the equipment manufacturers offer individual solutions. However, despite many small differences, all of the providers offer comparable approaches. After removing the glued-on glass, both Schmid and Decker machines take the uppermost wafer in the stack, place it on a conveyor belt and inspect it for breakage or other grounds for rejection, such as whether two wafers are stuck together. Defective wafers are immediately ejected; only the good ones stay on the conveyor belt. One characteristic of this process is the position of the wafers.
They are horizontal from when they are taken from the stack to when they are conveyed either to a buffer or a distribution system for fine washing. But while Schmidt loosens the wafer stack with a gentle flow of water and grabs the uppermost wafer with a belt, Decker uses an adhesion gripper of its own design. The gripper creates a thin cushion of water between itself and the wafer to ensure gentle conveyance.
The AMB and Rena singulation processes are also similar. Rena’s machine puts the wafers in a basket before the glass plate is removed. Brushes tightly hold the wafers to prevent them from falling out. The basket of wafers is then horizontally immersed into a water bath in the machine, so that they are next to each other rather than on top of one another. Following pre-singulation using a stream of water, a picking system lifts each wafer. Then, the fragile slices are tilted carefully into the horizontal position, inspected, and the rejects are ejected.
Rena then conveys the wafers off at a 90 degree angle and slides them into the lanes for the next processing step. A downstream distribution system fills the conveyor lines for fine washing.

Singulation from the bottom up

ACI-ecotec has its own philosophy. It uses synchronized, specially coated rollers to convey the wafers from the bottom of the stack to a metal barrier adjustable with micrometer precision, which allows only a single wafer to pass through. Mechanically defective wafers – broken, too thick, and doubled – are rejected while the rest lie flat on conveyor belts to a fine washing. The system does not monitor wafers for damage. That is left to an inspection system at the end of the line. Felix Jäger has a simple explanation for singulating wafers from the bottom, “It lets us work entirely without interruption.” However, loading stoppages in competitors’ machines are negligible, and they can also operate continuously if equipped with buffers.

Outcome unknown

Regardless of the process used, production costs can be reduced as long as the equipment works trouble-free. To save costs, the production line has to have a high annual throughput of wafers of a single type. While that limits the flexibility of the production line, automation companies still have to provide equipment with the greatest possible flexibility – sustainable equipment capable of converting to thinner or larger wafer formats in the mid-term without substantial investment. It is also important for singulation equipment to have straightforward connectivity and an adaptable controller for integration in a wide range of different production lines. Compatibility is especially important because customers increasingly want complete lines from a single source. “Then – and especially with turnkey systems – the equipment for each process step has to fit together seamlessly,” Nitz explains.
However, it is too early to conclude that only providers offering the entire process chain are ahead of the game. Rather, the need for good integration results in cooperation between equipment manufacturers. If manufacturers cooperate, the machines that win out will be those best suited to automated processes.