Feature: Sensors
Piezo dynamic force measurement in semiconductor manufacturing
By Robert Hillinger, Business Development Manager, Kistler Group T
he advance of 5G, IoT and Big Data, as well as new technology introductions, places greater demands on semiconductors. As application requirements
become more complex, so does the need for better-performing semiconductors, which in turn makes their production more challenging. Conventional approaches to process
control, such as optical and displacement sensors in combination with electrical testing, can’t keep up with these demands. However, measurement of dynamic force with piezoelectric sensors has proven an effective method to control production processes. Te applied process force is a critical factor in manufacturing semiconductors, both in front-end processes such as wafer grinding, polishing, CMP (Chemical Mechanical Planarisers) or delamination, and in back-end processes including lead-frame stamping, die, wire and wafer bonding, sealing, and sorting. Deviations in the applied force during these processes mean mechanical stress, which can lead to quality issues. Force measurement is
therefore an important method of achieving tighter process control and avoiding mechanical stress, caused, for instance, by tool wear, changing material behaviour or machine malfunctions during semiconductor production; it enables users to correlate force signals to certain product quality parameters.
Piezoelectrical force measurement Force measurement with piezoelectric sensors has shown to be a highly-efficient method for monitoring, controlling and optimising semiconductor production. Te operating principle here is based on the piezoelectric effect, where piezoelectric (PE) materials such as quartz crystals generate linearly-proportional electrical charge in response to pressure. High linearity means highly-accurate measurements, in the range of 0.1N to 100kN. Another advantage is that the PE effect
occurs in the direction of the force, and diagonally as a shear effect, allowing PE sensors to be integrated in various ways in an application. Due to their rigidity, they are highly responsive to rapid force changes. PE sensors are not used as single measuring
24 July/August 2021
www.electronicsworld.co.uk
units, but as a part of a measurement system. For example, a PE sensor measures the force, a charge amplifier converts it and provides the programmable logic controller (PLC) or industrial PC with a signal equal to the measured force. Soſtware represents each production step,
visualised by a curve (either force/time or displacement), making it easy to check for quality. Users can additionally adapt the curve evaluation to the individual monitoring task by using evaluation objects. With this approach, every production step can be monitored to determine whether the part is good or bad. Besides process-monitoring hardware and
soſtware, Kistler offers a broad range of PE sensors and charge amplifiers, so users can select the most suitable sensors according to the application and available space. Te best-fitting charge amplifier can be chosen depending on the number of channels, measurement range and type (static/ dynamic), analogue or digital output signal, and frequency range. Besides careful design of components of the measurement chain, good measurement practices include avoiding
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