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Compiled by George Allen
Photovoltaic solar cells’ role in producing power becomes more attractive as oil prices climb and governments use subsidies to promote less foreign oil dependency by accelerating the installation of solar panels. Solar industry manufacturers are hurrying to increase production as demand rises exponentially. But solar manufacturers are also working quickly to lower cell costs to approach the economics of conventional energy to insure their competitiveness. Solar cell production is driven by yield and throughput right now.
Automation is being integrated into the solar production process to help boost that increase in throughput. Automating solar wafer production has its own set of problems. Solar cell wafers used in series to make a solar panel are made with a similar production process to manufacturing semiconductor wafers. But one semiconductor trend in particular has been taken to the extreme in photovoltaic production: thin wafers.
[Excerpt A]
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 chip making. 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 air levitation 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 servo valves. 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 super thin.
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. [Retractable pins are one often-used solution.]
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 edge forward 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.
ICOS Vision Systems Product Manager Bruno Gouverneur says industrial vision systems frequently check for defects such as fingerprints, 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 micro cracks, 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.
…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.
… 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 super thin wafers,” says Steele.
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.
DWFritz Automation brings the needed expertise to integrate and modify new technologies such as Bernoulli (venture) end effectors and machine vision systems to overcome many solar production problems with sophisticated automation that works effectively, efficiently and increases throughput.
[Excerpt B]
Turmoil in PV manufacturing has brought a need for integrators experienced in automation. “PV makers have been in a rush to scale up production because they have pre-sold the inventory of every plant they can build,” says Rockwell Automation Industry Solutions Manager for Solar and Semiconductor Bates Marshall. …“PV thin-film people are focused on speed of integration, ramping up production, and expanding to support factories globally,” he says. “This contrasts with the semiconductor industry where there is still a great deal of invention going on in control systems because of a perception that more benefits can be had by inventing automation technologies.”
One factor complicating automation efforts in PV is that “In thin-film, manufacturing processes of different suppliers are completely unique. That’s why there are so many of them,” explains Marshall.
There is more commonality in crystalline production processes and today they are more mature than thin-film lines. “The layout of different PV module assembly lines would look a lot like the ones we sell,” says Spire Corp. Sales and Marketing Vice President Mark Willingham. Spire manufactures equipment for making PV modules as well as turnkey PV-module production lines. It says about 90% of all PV manufacturers have pieces of its equipment in place.
In the making of a crystalline-silicon solar module, the first difficulty manufacturers must overcome in automating their operations is handling the in-coming silicon wafers. Modern silicon wafers for solar cells are only on the order of 100 to 200 microns thick. Separating the thin, fragile silicon wafers from the top of stacks is tricky. The typical approach is to employ an air knife to separate wafers from the stack and either vacuum or venturi-type grippers for moving them around.
The thinner the wafer, the more the likelihood of damage during handling. There are efforts to cut costs by making wafers even thinner than 100 microns, but industry veterans doubt the practicality of such devices because of the challenges in handling they would entail. ”I’ve never seen one below 100 microns,” says Spire’s Willingham. “Some big manufacturers have announced 160-micron devices but I doubt they will get even this thin. And the cost of raw silicon is expected to drop. This will alleviate the rationale for thinner wafers.”
“We have seen sample wafers at 90 microns,” says NREL’s Mitchell. “Though it becomes a cost problem if the yields aren’t high enough.”
DWFritz Automation’s engineering team has designed custom, sophisticated wafer handling machines for semiconductor industry leaders for many years. More recent inquiries include solar panel producers wanting the throughput and cost savings discussed throughout this article. Our designs make use of Bernoulli (venture) end effectors to transport very thin wafers with minimal contact avoiding potential damage to the material.
Our experience in choosing the right vision system and integrating it into an effective component of the thin wafer production process insures trouble-free, efficient throughput. Our engineers work daily with the industries most exacting positioning and presentation problems. Our lighting and control of external factors insures the vision cameras can perform their intended role even when applied to difficult polycrystalline wafers used in photovoltaic production.
DWFritz Automation specializes in integrating intelligent machine vision, robotics, and micron-level precision technologies to solve the most difficult custom, thin wafer automation challenges.
[Excerpt C]
Increasing use of very thin, flexible wafers for IC manufacturing brings with it extremely challenging automated handling problems. Individually, vacuum- or Bernoulli-based end-effectors cannot solve all problems.
In today’s wafer-processing applications requiring wafer thinning, 150mm wafers may be thinned to a 50µm limit, 200mm wafers to 100µm. At these thicknesses, wafers are flexible, fragile objects. If a wafer bows from a backside metal, standard vacuum end-effectors or electrostatic handling fail, making routine handling a significant challenge.
Figure 1. End-effector that combines vacuum and Bernoulli wafer "gripping."
The instructions to "never handle a thin wafer without tape" cannot always be followed. First, no tapes are available that withstand high-temperature backend processes. Furthermore, carriers or protection tapes are not applicable before test because a wafer has to be uncovered on both the front- and backsides for electrical contact and visual inspection.
With the forecast growing demand for thin silicon wafers, it is often predicted that conventional wafer handling will not be able to achieved targeted wafer breakage rates. Currently, many thin-wafer-handling operations are carried out manually due to lack of automated handling systems; this explains high breakage rates.
[Excerpt D]
A device that supports a substrate in a non-containing manner so as to controllably move the substrate within a substrate processing system. The device includes an end effector carrying support pads each having a vertical gas outlet and a horizontal outlet communicating with a gas supply to form a plurality of vertical gas jets and a plurality of horizontal gas jets. The vertical gas jets impinge on a lower surface of the substrate to urge the substrate into a fixed vertical position with respect to the support pads and the horizontal gas jets impinge on the substrate edge to urge the substrate into a fixed horizontal position with respect to the pads.
[Excerpt E]
A new mechatronic approach to thin wafer handling and pre-aligning issues in the semiconductor fabrication.
Thin wafers are less than 200 µm thick and require specified handling solutions. One solution is the Bernoulli-Vacuum combined grip principle that successfully treats wafers with significant warpage up to 10 mm. A mechatronic gripper, which can replace standard pre-aligners by incorporating the functionality into the gripper and decreases cycle times is discussed. The Mechatronic Design and the control algorithms are explained in detail. Test results show that the accuracy at the system is 100 µm and the repeatability is 50 µm.
This paper describes the design and testing of a gripper developed for the handling of delicate sliced fruit and vegetable products commonly found in the food industry. The device operates on the Bernoulli principle whereby air flow over the surface of an object generates a lift. The gripper allows objects to be lifted with minimal contact thereby reducing the chances of damaging or contaminating the object.
An improved Bernoulli end effector for holding, handling, and transporting ultra-thin substrates includes edge guides to aid in the positioning of the substrate and may include friction pads that impede motion of the substrate lifted by the end effector. The Bernoulli end effector may be incorporated into an apparatus and method for supinating a substrate so that both surfaces of the substrate can be processed. In addition, the Bernoulli end effector may be used to place ultra-thin substrates on and retrieve substrates from a substrate handling structure that includes weights that prevent the substrates from bowing or flexing during processing and includes guides that prevent the ultra-thin substrates from moving or translating on the surface of the substrate handling structure.
A micro pneumatic end-effector for micromanipulation and micro assembly with in-situ PVDF sensing is designed and calibrated. The micro pneumatic end-effector system consists of a DC micro-diaphragm pump and compressor, two regions of flexible latex tubes with different parameters and different function such as micro tool and air channel. Effectively controlling the suction force and pressure are critical for the performance of the micro-pneumatic end-effector for micromanipulation and micro assembly. The force-sensing model was developed for the pneumatic end-effector system. An effective calibration method is proposed and its results verify the behavior of the developed pneumatic end-effector system. Ultimately the technology will provide a critical and major step towards the development of automated manufacturing process for batch assembly of micro devices, and also enhance micromanipulation
[Excerpt F]
In fluid dynamics, Bernoulli’s principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s gravitational potential energy. Bernoulli’s principle is named after the inventor Daniel Bernoulli.
Bernoulli’s principle can be applied to various types of fluid flow, resulting in what is loosely denoted as Bernoulli’s equation. But in fact there are different forms of the Bernoulli equation for different types of flow. The simple form of Bernoulli’s principle is valid for incompressible flows (e.g., most liquid flows) and also for compressible flows (e.g., gases) moving at low Mach numbers. More advanced forms may in some cases be applied to compressible flows at higher Mach numbers (see the derivations of the Bernoulli equation).
Bernoulli’s principle is equivalent to the principle of conservation of energy. This states that in a steady flow the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. If the fluid is flowing out of a reservoir the sum of all forms of energy is the same on all streamlines because in a reservoir the energy per unit mass (the sum of pressure and gravitational potential ρgh) is the same everywhere.
Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.
In every-day life there are many observations that can be successfully explained by application of Bernoulli’s principle.
The relative airflow parallel to the top surface of an aircraft wing or helicopter rotor blade is faster than along the bottom surface. Bernoulli’s principle states that the pressure on the surfaces of the wing or rotor blade will be lower above than below, and this pressure difference results in an upward lift force. If the relative air flows across the top and bottom surfaces of a wing or rotor are known, then lift forces can be calculated (to a good approximation) using Bernoulli’s equations—established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Note that Bernoulli’s principle does not explain why the air flows faster past the top of the wing and slower past the under-side. To understand why, it is helpful to understand circulation, the Kutta condition and the Kutta–Joukowski theorem.
Besides, Newton’s third law states that forces only exist in pairs, so the air’s upward force on the wing coexists with the wing’s downward force on the air, which results in a downward acceleration of air.
The carburetor used in many reciprocating engines contains a venturi to create a region of low pressure to draw fuel into the carburetor and mix it thoroughly with the incoming air. The low pressure in the throat of a venturi can be explained by Bernoulli’s principle - in the narrow throat, the air is moving at its fastest speed and therefore it is at its lowest pressure.
The pitot tube and static port on an aircraft are used to determine the airspeed of the aircraft. These two devices are connected to the airspeed indicator that determines the dynamic pressure of the airflow past the aircraft. Dynamic pressure is the difference between stagnation pressure and static pressure. Bernoulli’s principle is used to calibrate the airspeed indicator so that it displays the indicated airspeed appropriate to the dynamic pressure.
The flow speed of a fluid can be measured using a device such as a Venturi meter or an orifice plate, which can be placed into a pipeline to reduce the diameter of the flow. For a horizontal device, the continuity equation shows that for an incompressible fluid, the reduction in diameter will cause an increase in the fluid flow speed. Subsequently Bernoulli’s principle then shows that there must be a decrease in the pressure in the reduced diameter region. This phenomenon is known as the Venturi effect.
The maximum possible drain rate for a tank with a hole or tap at the base can be calculated directly from Bernoulli’s equation, and is found to be proportional to the square root of the height of the fluid in the tank. This is Torricelli’s law, showing that Torricelli’s law is compatible with Bernoulli’s principle. Viscosity lowers this drain rate. This is reflected in the discharge coefficient, which is a function of the Reynold’s number and the shape of the orifice.
A - [Excerpt from ‘Next Big Challenge for PV Makers: Wafer Handling’: The push to crank out solar cells more quickly brings problems as manufacturers work with ever-thinner silicon wafers.] B - [Excerpt from Solar Production Shifts Into High Gear: Solar makers see automation as a strategy for becoming more competitive with fossil fuels. ] C–[Excerpt from ‘Novel technology for handling very thin wafers’ - Alfred Binder, Gerhard Kroupa, Carinthian Tech Research AG, Villach St. Magdalen, Austria Reprinted excerpts from Solid State Technology web site owned by Penwell. D–Excerpt from ‘US Patent 6322116 - Non-contact end effector’ E–Excerpt from ‘Thin Wafer Pre-Align Gripping System–A new mechantronic approach to semiconductor handling’ - A. Binder, G. Franz, M. Lenzhofer, M. Soucek F - Excerpt from Wikipedia G–Graphic from ‘A classification scheme for quantitative analysis of micro-grip principles’ - Authors: Marcel Tichem, Defeng Lang, Bernhard Karpuschewski H–Graphic from Technology Zones blog by By Gene Gautro for Hydraulics & Pneumatics
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