Robotics for positioning wafers for PECVD coating

Image processing-supported robotics enables micrometer-precise positioning in the PECVD coating of wafers. This has made it possible to automate the loading and unloading of the workpiece carriers.

For the automated loading and unloading of a workpiece carrier with wafers of different sizes prior to PECVD coating, Acp Systems developed an image processing-supported robotic solution for a leading manufacturer of space solar technology. It ensures that the specified positioning accuracy in the nests of the workpiece carriers of +/- 0.1 mm is maintained and that both the manufacturing tolerances of the carriers and their shrinkage caused by cooling during loading are compensated for.

Heilbronn-based Azur Space Solar Power GmbH is one of the world's leading companies in the development and production of high-efficiency multi-junction solar cells for space and terrestrial concentrator systems (CPV). The solar cells are based on the latest triple and quadruple junction technology, in which the layers are built up on a germanium substrate.

During the manufacturing process, the 4-, 6- and 8-inch (100, 150 and 200 mm) diameter wafers undergo a PECVD (plasma-enhanced chemical vapor deposition) process in Singulus Technologies AG equipment. The solar cells are provided in cassettes, removed and positioned in the pockets of special carbon fiber workpiece carriers, which are only a few hundred micrometers larger. Depending on the cell size, the 1000 x 600 mm carriers can hold four, nine or 16 wafers. To avoid crashes, a stable positioning accuracy of +/- 0.1 mm must be maintained when loading the workpiece carriers. After coating on one or two sides, the solar cells must be placed back into cassettes.

Azur Space wanted to automate this previously time-consuming and cost-intensive manual process using suction pipettes. Challenges arise from the position of the solar wafers with flats in the cassettes with deviations of +/- 5 degrees and +/- 3 mm as well as precisely specified positions for gripping. In addition, the production-related tolerances of the carriers must be compensated for, as must the shrinkage caused by cooling. This results from the falling temperature of the workpiece carriers, which come out of the coating process at up to 350 °C and cool down during unloading and loading.

For this task, automation specialist Acp Systems AG developed an intelligent, image processing-supported handling solution with an industrial robot. For space reasons, this was mounted on the ceiling of the loading area of the coating system and has a reach of 1,000 millimetres. The Scara is equipped with a special flat vacuum gripper system that can be quickly exchanged for wafers of different sizes.

The robot removes the wafer from the cassette and places it on a backlit alignment table. A camera system with a 12-megapixel camera is positioned above it at a working distance of 680 mm. It detects the exact position of the wafer and passes this information on to the Cognex Vision Pro software. Based on this, the position and angle compensation with which the wafer must be inserted into the carrier nest is calculated and transmitted to the robot controller. Any distortions in the camera system were compensated for during commissioning by calibrating it with a checker plate.

In order to control the manufacturing tolerances of the carriers and the shrinkage caused by cooling, the workpiece carrier is first centered by pulling it against a stop and indexing it. This allows the coordinate zero point of all carriers in the handling system to be reproducibly defined. In addition, to compensate for manufacturing tolerances, all carriers were measured precisely beforehand in their cold new condition and each was given a data matrix code for identification. This code is used to store relevant data in the control system for calculating the compensation of the position tolerances of the carrier nests.

In order to compensate for the thermal shrinkage caused by the cooling of the workpiece carriers, a fiducial mark was first placed in the corner of the carrier opposite the coordinate zero point and this was also measured precisely when cold. A second camera system is located above this, which is used to determine the offset of the fiducial mark compared to the cold state. The software uses this information to calculate the compensation for the exact positioning of the wafer. This process is repeated for each wafer to be inserted.

Acp Systems has integrated a flipping station for turning the solar cells, which are coated on both sides. This receives the corresponding wafers individually from the robot and grips them at defined areas with vacuum suction points. After rotation by 180 degrees, the robot gripper picks up the wafer again and transports it to the alignment table.

Before the coated solar cells are placed back in the cassettes, a final quality check is carried out by the camera system on the alignment table. This checks whether the edges of the wafers are free of damage.

The image processing-supported robotic solution described above ensures highly precise and gentle handling of the very sensitive solar wafers. This is demonstrated above all by the fact that there have been no handling-related wafer breakages since commissioning. Overall, the replacement of manual handling with a fully automated system has resulted in significantly improved productivity and efficiency (OM-6/25).