Piezodynamic Force Measurement Improves Manufacturing of Semiconductors

New technologies like 5G or IoT and progress in Big Data handling have increased the demands on the performance of semiconductors. As the requirements are becoming more complex, so is their production. As a result, maintaining or improving the quality and yield of production and packaging processes have become even more challenging. Conventional approaches to process control – such as using optical and displacement sensors in combination with electrical testing – cannot keep up with these demands. Here, measurement of dynamic force with piezoelectric sensors has proven to be an effective method to control production processes. A closer look at how piezoelectric sensors work shows how they help overcome the challenges in semiconductor production.

The applied process force is a critical factor in manufacturing semiconductors, both in front-end processes such as wafer grinding, polishing, CMP or de-lamination, and in back-end processes including lead frame stamping, die, wire and wafer bonding, sealing, or sorting. Deviations in the applied force during these processes mean mechanical stress and can lead to quality issues. Therefore, force measurement is an essential 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 specific product quality parameters.

Capabilities of piezoelectrical force measurement

In the past years, force measurement with piezoelectric sensors has shown to be a highly efficient monitoring method, controlling and optimizing semiconductor production. The operation principle is based on the piezoelectric effect, which postulates that piezoelectric (PE) materials such as quartz crystals generate an electrical charge signal in response to a mechanical load. Most importantly, the charge is linearly proportional to the applied force. The high linearity allows for highly accurate measurements in wide measuring ranges: the high-resolution sensors are designed for measuring ranges from 0.1 N to 100 kN. Another advantage is that the PE effect occurs in the direction of the force, in relation to the force and diagonally as a shear effect. Therefore, PE sensors can be integrated into a machine in various ways. Due to their rigidity, they are highly responsive to rapid force changes. Overall, its features make PE sensors highly suitable for many challenging measuring applications.

The piezoelectric measuring chain: proceedings and device selection

PE sensors are not used as single measuring units; and instead, they are part of an industrial measuring chain into which they can be seamlessly integrated. In such a chain, the PE sensor measures the force. A charge amplifier converts it and provides the programmable logic controller (PLC) or industrial PC with an electric signal equal to the measured force (see figure 1).

Figure 1: PE load washer sensor with charge amplifier and programmable logic controller

With the help of process monitoring hardware and software such as Kistler’s, the measured signals are processed further. Each production step is visualized 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 checked to determine whether the part is bad or good. Besides process monitoring hardware and software, Kistler offers a broad range of PE sensors and charge amplifiers. Users can select the most suitable sensors according to the application and available space. The best-fitting charge amplifier can be chosen depending on the number of channels, measurement range, and type (static/dynamic), analog or digital output signal, as well as the 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 they cannot be avoided, it is necessary to implement measures to overcome their influence. Furthermore, users should provide good cable insulation and route and 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 – offers essential possibilities. Together with the charge amplifier and process evaluation unit, inline sensors enable users to record and assess the applied force. Last but not least, they can optimize 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 the automatic inspection of mass-produced items. This also entails traceability for each produced item.

Achieving accurate measurement results

Resolution is defined 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 1 N, PE force measurement is able to achieve resolutions of less than 0.01 N in industrial applications. In addition, PE measurement comes up with quality levers such as repeatability or serial precision, which determines the accuracy of repeated measurements between identical production steps. Users can take advantage of this and use relative instead of absolute measurement.

Force measurement in semiconductor processes

In the past, piezoelectric force measurement technology was mainly used for machine verification 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 (see table 1). The complexities of semiconductor production processes will increase even further in the future and require significant effort to improve the quality and yield of manufacturing. Dynamic piezoelectric force measurement is the technology of choice for meeting these requirements, offering high resolution and repeatability even for low forces. Manufacturers profit from the sensors’ long lifespan and compact sizes. The benefits for the production itself are manifold: it improves machine performance regarding speed and accuracy, increases the quality of the produced items, thus reducing the ppm failure rate, and ultimately decreases costs.