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NDSU Researchers Develop Method for Embedding RFID in Paper
The North Dakota State University group claims that its laser-based RFID inlay assembly method can be used to inexpensively incorporate tags into paper items, such as tickets and banknotes.
LEAP employs a laser beam's energy to precisely transfer and assemble semiconductor die and other discrete components with dimensions well below those possible using the conventional robotic ("pick-and-place") methods. The method involves the use of a multilayered bonding material, known as the dynamic release layer (DRL), deposited on its surface. The RFID wafer is first thinned to the desired thickness, and is then bonded to the DRL. During the next step, the thinned wafer is diced into individual RFID chips (dies) using plasma etching. Separately, the antenna and pads for joining the antenna to the die are printed onto a paper substrate, with a layer of anisotropic conductive adhesive applied to antenna pads.
Once the inlay is ready for assembly, an ultraviolet (UV) laser pulse is directed at the DRL bonding material holding the die, thereby creating a blister. The force exerted by the blister causes the contactless die to transfer from the wafer onto the receiving substrate (the system includes a camera that allows for proper alignment of the laser beam), where the chip is connected to the antenna pads. The substrate can then be laminated to a second sheet of paper, thereby embedding the chip and antenna in the paper, which can then be used to manufacture banknotes or other paper items.
Alien Technology Higgs 3 chips that they mechanically ground down to a thickness of approximately 65 microns (0.003 inch); at the time, the group had not yet mastered the process of thinning beyond 65 microns. During a second test, however, they ground a wafer of uncut dummy silicon chips (not RFID ICs) down to 50 microns (0.002 inch) in thickness, an then further thinned them to 20 microns (0.0008 inch) via plasma etching (the second part of the thinning process was performed with etching in order to minimize the risk of damaging the chip).
During both tests, the wafer was then bonded to the carrier adhesive, and was cut into chips that were transferred to the paper substrate using the LEAP laser-based method. The substrate containing the chip and antenna was embedded in paper the size of a banknote. The paper was then read using an Alien ALR-9650 RFID reader. Nine of 10 samples could be interrogated using the reader, Marinov says.
Not only is the LEAP manufacturing process more precise, Marinov says, but it can also be performed much more quickly than traditional pick-and-place methods. He estimates that packaging chips on substrates can be completed at least twice as fast as is possible via traditional methods, since multiple dies can be transferred almost simultaneously in parallel to a single large sheet of paper by moving the laser beam very quickly over each chip to be transferred. The large paper sheet would then be cut into individual pieces, each embedded with its own RFID chip and antenna. The increase in throughput, he notes, can be achieved without increasing equipment expenses.
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