Robotic Automation for Solar Cell Manufacturing

Rush Laselle, !%Adept Technology Inc.%!

The global photovoltaic (PV) manufacturing community is on the cusp of a resurgence in investment, development and innovation, a revolution that largely will be driven by technology. It is vital to find the most effective – and most cost-effective – tools and processes to increase productivity and decrease costs within a set capital plan. Robotic automation is a significant part of solar cell manufacturing, but it is important to consider which robot types and kinematics are best for each unique process, looking at the solar manufacturing areas where there are the greatest return opportunities for robotic automation as well as investigating which robot type is best for a particular solar application task and where vision fits.

In this article, I will provide a primer targeting these issues and discuss how the solar industry can best maximize factory throughput, reduce costs and improve efficiencies with robotic automation.

Flexibility through vision

Vision has become a highly adopted tool to improve the productivity of robotic automation in all industries and all facets of placement. Vision systems offer tremendous flexibility for applications that don’t require fixtures or trays for part location. Vision guidance enables the system to take a picture, compute a part’s location and orientation, and guide the robot to the part using a computed robot-to-camera transformation obtained through an automated calibration process. Tremendous flexibility and cost savings are realized because parts don’t have to be fixtured.

Vision systems allow parts to be randomly presented to the robot without orientation or alignment, or without being placed in a tray, which also reduces cost. These systems frequently incorporate line tracking, enabling the robot to pick these parts from a moving belt, further optimizing the process.

Robot-integrated vision allows inspection to be incorporated right into the handling process, placing it in parallel with handling, further reducing the overall cycle time and increasing throughput. Various part geometries require only vision retraining or the selection of a recipe instead of manual changes in fixtures and tooling. This increases the overall lifetime profit of the equipment by virtue of its optimization and improved throughput. Most robot manufacturers offer packages with multiple cameras and tracking solutions for integration into a single cell. This offers tremendous power and flexibility for solar manufacturing.

The right kinematic solution

How do you select the right robot for the task? First, you must consider the payload requirement for the robot. People often consider only the products being handled. But it is also important to consider the tooling solution or end-of-arm tooling (EOAT).

Evaluating the motion requirements also is critical. Not only the simple motion of picking and placing but also what interferences exist between the robot and its linkages as well as other items that may be in dynamic motion within the cell must be considered.

How repeatable must the robot be? Robot manufacturers speak in terms of repeatability, while engineers and designers look at accuracy. A robot’s repeatability outlines the machine’s ability, once programmed, to return to the taught position. Accuracy refers to the ability to input a given location digitally and to have the robot move to that point in space “accurately.” This encompasses offsets and other digitally input motion parameters, often varying within a given mechanical unit’s work envelope. Thus, you must make sure you carefully evaluate these factors to gain a good understanding of the requirements of a process plus the capabilities of a given robotic solution.

Do your processes require special environmental considerations? Do you need a robot designed to eliminate the generation of particulates that might degrade the product? Or must the robot be protected from process-specific elements such as those in slurry ingot processing?

Robot kinematics

The four major categories of robot kinematics are Cartesian, SCARA (selective compliant assembly robot arm), articulated and delta/parallel.

The Cartesian kinematic solution is simple and highly configurable (Figure 1). The platform includes everything from a single degree of freedom or unidirectional travel to numerous axes of motion. Adjusting strokes or lengths and configuration is relatively easy with Cartesian kinematic solutions as compared with the other types. Multiple drivetrains exist, optimized to provide high throughput or precise motion as characterized by whether the drive might be a ball screw or a belt-driven mechanism.

Figure 1. Cartesian solutions are typically called upon to serve applications where the substrate remains in the same plane. Images courtesy of Adept Technology.

Platforms exist to accommodate small-part assembly up to extremely large part transfer such as overhead cranes that might be observed in a manufacturing facility. In the photovoltaics industry, Cartesian solutions can be applied to both small and large work spaces. They typically serve applications where the substrate remains in the same plane and does not have to be flipped or have its configuration changed, other than a rotation in the same plane as the table or conveyor (X-Y plane).

Figure 2. Within solar manufacturing processes, SCARA robots are best suited for high-speed and high-repeatability handling of cells in smaller work spaces.

The next robot is the SCARA, which offers a cylindrical work envelope and typically provides higher speeds for picking, placing and handling processes than do Cartesian or articulated robotic solutions (Figure 3). It also delivers greater repeatability by offering positional capabilities superior in many cases to articulated arms. This class of robot usually is used for lighter payloads in the sub-10-kg category for assembly, packaging and materials handling. For solar manufacturing processes, it is best suited for handling of cells in smaller work spaces with high speed and repeatability.

Figure 3. With more automation, the solar industry can potentially realize a 75 percent savings in direct labor costs alone. Courtesy of International Federation of Robotics.

Articulated robots, the third group, have a spherical work envelope, and their arms offer the greatest level of flexibility because of their articulation and higher degrees of freedom (Figure 4). Because they make up the largest segment of robots available, you can find a wide range of solutions, from tabletops to large 1000-kg-plus solutions. Articulated robots are applied to a variety of solar applications, including the handling of heavy silicon ingots or glass, or the handling of subassemblies or assemblies where the products are introduced to the cell in a configuration different from the way in which they are presented to the system.

Figure 4. Articulated robots are frequently applied to process intensive applications where they can use their full articulation and dexterity for solar applications such as handling silicon ingots, glass, subassemblies and assemblies.

Delta/parallel robots make up the fourth category (Figure 5). This kinematic solution provides a cylindrical work envelope and is most frequently applied to applications where the product again remains in the same plane from pick to place. The design uses a parallelogram and produces three purely translational degrees of freedom, so work must be done within the same plane. Base-mounted motors and low mass links allow for exceptionally fast acceleration and, therefore, greater throughput over the other groups. An overhead-mounted solution, it maximizes access but also minimizes footprint. These units are designed for high-speed handling of lightweight products and offer lower maintenance because of the elimination of cable harnesses and cyclical loading.

Figure 5. Parallel robots offer high-speed transfer of solar cells through manufacturer lines and a multitude of processes.

Parallel robots are deployed into many solar cell processing steps because they offer high-speed transfer of solar cells through manufacturer lines and a multitude of processes, including diffusion of process equipment, wet benches and PECVD (plasma-enhanced chemical vapor deposition) antireflection coating machines. The Quattro parallel-linked product from Adept Technology Inc. recently achieved 300 cycles per minute, illustrating the capabilities for this class of machine to handle products at high rates.

Deployment within the solar process

Typical PV process steps can be seen in Figure 6. The steps are broken into four basic groups where high concentrations of robots are deployed. The ingot processing step predominantly uses Cartesian gantries and large articulated arms because of the requirement for heavier payloads and large work-space optimization. Wafer manufacturing uses a variety of arm types, depending upon volume and process requirements. Cell processing tends to use gantries, SCARAs and parallel linked robots. Reach and repeatability considerations usually are the deciding factors. Module building uses a variety of arms – often articulated and Cartesian arms for reach and flexibility, although some specific tasks use the services of SCARAs and parallel robots.

Figure 6. High concentrations of robots are being deployed into the four basic photovoltaic process steps.

Antireflection coating process

Comparing the four robot categories, we consider their usefulness for an antireflection coating load/unload process. A Cartesian robot is optimized from a reach standpoint. However, the majority of solutions here would prove too slow and would require in excess of a single-head EOAT tooling. Because this complication would drive the need for prealignment and could result in further complications in preconditioning the product, a Cartesian solution could be considered less flexible.

Cartesian robots

• Optimized for reach

• Too slow for loading/unloading using a single-head EOAT

• Because multihead EOAT often is used, cells require prealignment.

• Less flexible when reconfiguring for different size wafers is required

SCARA robots enable increased speeds and are more flexible than Cartesians. However, in a traditional tabletop version, the work space is limited, so SCARA may not be optimal in reaching all points on the load and unload areas.

SCARA robots

• Faster and more flexible than Cartesian robots when used with vision guidance

• Table-mounted versions could limit work space, and multiple robots may be required to cover pallet/matrix.

Articulated robots would be pedestal-mounted and could prove too slow in increasing the complexity of the installation.

Articulated robots

• Too slow for loading/unloading with single-head EOAT

• Spherical work envelope isn’t ideal for covering pallet/matrix.

The optimal choice might be a delta or parallel-style robot, for a number of reasons. The overhead mount is ideal for reducing the footprint of the automation cell. It can reach all places on the PECVD pallets. And when the benefits of the delta are combined with vision, it provides an exceedingly flexible solution that will meet the throughput requirements. Note: Vision is an enabler not only for parallel linked robots but for all categories of robots.

Delta/parallel robots

• Overhead mount design is ideal for loading/unloading equipment.

• Larger delta robots can cover the width of most PECVD pallets.

• When used with vision guidance, they enable extremely good positioning.

• Excellent flexibility and quickly reconfigurable

• Robot design is optimal for handling cells (lightweight) at high speeds.

Conclusion

The common goal for solar manufacturers is to drive down the cost per watt. As the solar industry strives to achieve grid parity, manufacturers must be knowledgeable about modern robotics as well as about automation and vision technologies – and the value they contribute to helping reduce the cost of solar cells.

History has shown that automation plays a significant role in reducing manufacturing costs in many industries, and when the costs associated with higher quality and yields are considered, its benefits offer an even more appealing value proposition. Although robotics and automation may be viewed by some industries as mature technologies, industry leaders are continuing to develop innovative products and new technologies that are ideal for solar manufacturing processes.

It would be prudent for solar manufacturers to look outside of their industry for the best practices in high-volume manufacturing with automation, robotics and vision to achieve cost-reduction goals.

Meet the author

Rush LaSelle is director of worldwide sales and marketing at Adept Technology Inc. in Pleasanton, Calif.; e-mail: rush.laselle@adept.com.