Driving Speed Wafer Testing with Fast Settling Servo Drives

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Fabricating semiconductors can be very complicated, with time-consuming steps and expensive equipment. The microelectronics industry can provide significant computing power at more reasonable pricing due to batch processing.

Batch processing is where hundreds of identical chips can be created simultaneously on large wafers. Component testing is essential in this process to support quality assurance and standards compliance.

Proaut Technology GmbH wanted to upgrade its PMA wafer testing series, intending to achieve a throughput of 40,000 components per hour. They did this by using the high-efficiency and high-performance servo drives from Elmo Motion Control.

Before calculations can start with the PMA, the wafer or panel must be installed in the test chamber. The PMA handling system utilizes a vacuum chuck to take away the component carrier from a magazine located next to the unit and places the carrier on a rotary positioning stage.

The exact measurement system comprises of the stage, a vision system to give feedback on the wafer position, and electronic and optical measurement tips to gather the actual test data. The stage is capable of nanoscale positioning in X, Y, and Z, and positioning in ф to a fraction of a degree.

The camera can be located along the Z axis for autofocus. During the process of measurement, the system tests segments of the patterned wafer in sequence, collecting measurements, and then advancing and aligning the wafer to place the next area underneath the measurement tips.

The problem with fast move-and-settle positioning is that inertia can lead the load to overshoot and oscillate about the commanded position (due to the high accelerations and small increments). This has the outcome of either introducing errors in calculation or making the measuring process inefficient.

The beginning PMA design had a throughput of an astounding 30,000 components per hour. To reach their target throughput of 40,000 components per hour, the Proauct engineering team needed a method of getting higher performance out of their existing mechanics.

They found a solution in the Elmo Gold Whistle family of servo drives. Gold Whistle drives contain a function referred to as Gain Scheduling by Profile Status.

In gain scheduling, an algorithm allows the system to categorize individual sets of controller parameters such as proportional gain (KP), integral gain (KI), higher-order filter links, and more, to enhance system performance for each of the many distinct application conditions.

In operation, the drive can instantly change from one set of controller parameters to another depending on the working point of the servo drive. The filtering assists in smoothing the effects of changing controller parameters which results in quicker settling.

Efficient and Compact

The Gold Whistle servo drives offered many other benefits. The application incorporates motion that is highly coordinated. The wafer handling axis must cooperate with the positioning stage and the ф axis of the positioning stage needs to be tightly coupled with positioning in X, Y, and Z.

This kind of motion control needs accurate and high-speed fieldbus connectivity. Gold Whistle drives are available with a selection of EtherCAT or CANopen. The super fast cycle times and reduced jitter of these protocols allow for world class real-time performance.

Reaching this degree of coordination demands a centralized control architecture utilizing a master controller to manage the operation of the drives for the separate axes. Drives and controllers are positioned in a centralized cabinet. The method makes integration and wiring more simple, but it does present challenges.

Packing six drives into a compact enclosed space locks heat that could possibly build up over time to lead to limited performance and the premature failure of the electronics.

In the semiconductor industry, downtime can cost hundreds of thousands of dollars each hour, so the team needed to mitigate this issue. Gold Whistle drives function with a wall-plug efficiency of over 99%, which reduces the amount of heat that needs to be dissipated.

Along with the efficiency of the equipment, the drives are also an ideal fit, literally. Their compact size allows them to be used in a smaller cabinet, thereby reducing machine footprint.

The PMA machines are very compact, with a small internal switch cabinet,” states Thiol Wicht, Managing Director of Proaut Technology GmbH. ”The ‘mini’ size of the Gold Whistle servo drive is very beneficial to our latest technology machines."

Gold Whistle drives also have the function of onboard intelligence. This helps built-in safety functionality, for example safe torque off (STO, SIL-3, IEC 61800-5-2). STO allows the drive to stay energized while taking torque away from the axis.

The combination speeds up restart while protecting the safety of the product, operators, and equipment. Lastly, the Elmo Application Studio (EAS) brings a unified software environment for servo motors and drives that makes it easy to adjust the system to variations in process or product.

With very fast settling times provided by gain-scheduling technology, Elmo’s Gold Whistle drives helped Proaut to increase the throughput of its latest-generation semiconductor test platform by 30%. Its 99% efficiency drives and compact form create a solution that is highly reliable.

The Proauct engineering team was highly satisfied with the performance. "The servo controllers of Elmo Motion Control fully meet all the criteria for our application,” states Mr. Walsh. “They are ruggedly constructed and predestined for use in our precision measurement machines.”

This information has been sourced, reviewed and adapted from materials provided by Elmo Motion Control Ltd.

For more information on this source, please visit Elmo Motion Control Ltd.

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