FEATURE Sensors & Sensing Systems
Piezo
dynamic force measurement for improved
production Robert Hillinger, Business Development Manager at Kistler Group, explains that measurement of dynamic force with piezoelectric sensors has proven to be an effective method to control production processes
T
he advance of 5G, IoT and Big Data as well as other new technologies, places greater demands on the performance
of devices. As application requirements become more complex, so does that of devices production. As a result, maintaining or improving the quality and yield of production and packaging processes has become even more challenging. Conventional approaches to process control, such as using 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 to be an eff ective method to control production processes. The applied process force is a critical factor in manufacturing semiconductors, both in front-end processes such as wafer grinding, polishing, CMP 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 parameter for achieving tighter process control and avoiding mechanical stress caused, for instance, by tool wear, changing material behavior and machine malfunctions in semiconductor production processes. It enables users to correlate force signals to certain product quality parameters.
Piezoelectric force measurement With the help of process-monitoring hardware and software, measured signals are processed further. Each production step is visualised by a curve (either
28 July/August 2021 | Automation
force/time or displacement), making it very easy to check for quality. Users can additionally adapt the curve evaluation to individual monitoring tasks by using evaluation objects. With this approach, every production step can be checked to determine whether the part is good or bad. Besides process monitoring hardware and software, Kistler off ers a broad range of PE sensors and charge amplifi ers. Users can select the most suitable sensors according to the application and available space. The best-fi tting charge amplifi er can be chosen depending on the number of channels, measurement range and type (static/dynamic), analogue or digital output signal, as well as frequency range. Besides a carefully-chosen design and components of the measurement chain, good measurement practices include avoiding acceleration and temperature changes during measurement. If these can’t be avoided, it is necessary to implement measures to overcome their infl uence. Furthermore, users should provide good cable insulation and routing, as well as compensate the reset/operate jump, for instance with the maXYmos evaluation system from Kistler or a PLC. Inline measurement – i.e., when force sensors are mounted in the mechanical structure of the semiconductor equipment – off ers important possibilities. Together with the charge amplifi er and process evaluation unit, inline sensors enable users to record and assess the applied force. They can optimise the production process based on the collected assembly data. As inline measurement chains allow for measuring and recording the force of every production step; they are suitable for automatic inspection of mass-produced
items, which also includes traceability for each produced item.
Accurate measurements Resolution is defi ned as the ability of the measurement system to detect and faithfully indicate small changes in the characteristic of the measurement result. As mentioned above, PE sensors achieve high resolution by nature. Even with forces as low as 1N, PE force measurement can achieve resolutions of less than 0.01N. In addition, PE measurement comes up with quality parameters such as repeatability or serial precision, which help determine the accuracy of repeated measurements between identical production steps. Users can take advantage of these features and use relative instead of absolute measurement.
In the past, piezoelectric force
measurement technology was mainly used for machine verifi cation and highly- accurate wire bonding, wafer grinding and polishing. Today, the technology is used in a growing number of applications throughout the semiconductor industry – in front-end and back-end processes as well as in testing processes such as sorting and taping. Manufacturers profi t from these sensors’ long lifespan and compact sizes. The benefi ts they bring to the production itself are manifold: improved machine performance regarding speed and accuracy, increased quality of produced items, reduced ppm failure rates and, ultimately, lowered costs.
CONTACT:
Kistler
www.kistler.com
automationmagazine.co.uk
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