Technological progress and the demand for smaller and more powerful processors lead to demanding requirements. New technologies, such as the stacking of very thin chips, are required to enable the manufacture of innovative products. The properties of these thin wafers and chips in particular pose an additional handling challenge. The materials are flexible, fragile and wavy, and they often have specially treated surfaces (e.g. with adhesive).

Some handling systems can present problems, such as contamination by particles in air bearings caused by the turbulence of compressed air. Other systems without air supply, such as vacuum grippers, can leave marks on the substrate during handling.

With the help of the patented ultrasonic bearing from ZS-Handling, substrates can float evenly on an air film generated by vibrations and thus be held without contact during handling. Thanks to a combination of negative pressure and ultrasound, attractive and repulsive forces act simultaneously on the workpiece, keeping it at a distance even when gripping from above.

How does the ultrasonic bearing work?
The ultrasonic movement of the so-called sonotrode creates a supporting gas film (air or process gas) between the sonotrode surface and the substrate. The substrate floats on the resulting gas film at intervals of ten to 150 micrometers. By utilizing buoyancy forces through vacuum, handling from above is made possible. In this way, any mechanical surface contact is avoided.

The physics of the ultrasonic bearing results from fluid dynamics and not from acoustic principles. The gas pressure in the gap between the workpiece and the vibrating surface increases due to the cyclical compression and decompression of the thin gas film. It is therefore necessary to realize a uniform vibration pattern in order to generate constant floating forces over the entire horn. The vibrations are not transmitted into the substrates and do not impair the substrate material.

With the repulsive forces of the ultrasonic bearings, the substrate can be moved without any friction, even at very high speeds. In addition, flexible materials can be "smoothed" without contact using this technology, i.e. held in a uniformly centered position. With very small components (up to around twelve by twelve millimetres), the self-centring effect works when the gripper tip is precisely adjusted, which means that no edge stops are required even for very precise positioning.

The force profile of the handling systems from ZS-Handling is similar to that of a conventional air bearing, but no compressed air supply is required. This means that in a clean room environment, the laminar air flow - unlike with Bernoulli grippers - is not disturbed by high flow velocities and no particles can penetrate via pipes due to external air. In addition, the costs for the compressed air supply can be saved in new production lines.

Floating substrate © ZS handling

By avoiding surface contact and without dynamic turbulence in the ambient gas, no damage, micro-scratches, micro-cracks or impurities can damage the substrate. Handling on processed or coated surfaces is possible without contact, allowing more degrees of freedom in process and machine design. A high degree of substrate evenness is also achieved during handling.

The systems can be used in all atmospheric processes and in up to 20 percent partial vacuum processes. This requires fewer resources, such as energy or compressed air, than a standard air storage system. This has a positive effect on the energy and cost balance for the handling systems from ZS-Handling.

Exemplary requirements of an application
In an ISO 6 cleanroom environment, dies of different sizes (ten by ten millimetres, five by five millimetres and five by twelve millimetres) are to be separated from a cut wafer (ø 300 millimetres) stapled onto sawing foil using a non-contact gripper. The dies are then to be deposited again for the subsequent bonding process.

The solution from ZS-Handling includes the MicroLevi gripper with different gripper tips for the various sizes, which can be easily exchanged using a collet chuck. With ultrasound and a simultaneous vacuum, the die can be gripped from above and held and swiveled without contact. Thanks to the self-centering effect of one gripper tip, the die can be gripped and deposited precisely even with rapid accelerations of over 2g. In order to achieve an optimum centering effect, the gripper tip must be precisely adapted to the format of the die, which is why three different gripper tips are required for this application. The gripper can be connected to any robot or axis system using an adaptable flange. The gripper and housing are made of anodized aluminium, the gripper tip is made of titanium.

In the application described here, the dies to be separated are pressed upwards by pins under the wafer in order to reduce the adhesive force on the adhesive film and facilitate contactless separation. The lifted die is picked up by the gripper tip without contact and transported at high speed to the next process step, where it can be deposited easily and precisely by switching off the vacuum.

The entire process for handling dies is fully automated. With the help of ultrasonic vibrations, the die is kept in a centered position on the uniformly generated air film, so that scratches or any other contact is avoided. In addition, the laminar air flow in the cleanroom is not affected by turbulence, which prevents contamination from particles. as