Microelectronic Engineering, Volume 101, January 2013, Pages 23-30.
Val R. Marinov, Orven Swenson, Yuriy Atanasov, Nathan Schneck
Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND 58102, USA
Laser-assisted assembly of ultrathin, ultrasmall bare dice holds promise for enabling a new class of very low cost flexible electronic devices with applications in many areas. But this advanced packaging technology will be of little use if two important parameters are not considered – transfer rate and precision/accuracy of die placement. Both parameters depend on the laser process parameters and the properties of the consumable materials used. Reported are results from a process study designed to investigate the effects of the laser pulse parameters, the adhesive layer properties, and the method of wafer dicing on the transfer rate and placement precision/accuracy when transferring ultrathin (50-um thick) silicon dice using the thermo-mechanical selective laser-assisted die transfer (tmSLADT) technique developed by this team and reported in prior publications. It is shown that, when properly controlled, tmSLADT can transfer ultrathin bare dice with precision and accuracy compatible with those achievable by the conventional die placement methods but at a much higher transfer rate.
The LEAP (Laser-Enabled Advanced Packaging) process developed at North Dakota State University is a comprehensive, wafer-to-product technique, based on using the energy of a carefully controlled and confined laser ablation to precisely and accurately transfer and assemble semiconductor die and other discrete components with dimensions well below those possible with the conventional robotic (“pick-and-place“) methods. LEAP is arguably the only electronics packaging technology with a potential of bringing the production cost of RFID tags and similar ultra-low cost products down to several cents while supporting packaging rates several times higher than those possible with the conventional pick-and-place equipment.
LEAP is a modification of the so-called Laser-Induced Forward Transfer (LIFT) technique. The basic concept of LIFT begins with a laser-transparent carrier substrate, which has a sacrificial layer deposited on its surface. The components to be transferred are bonded to the sacrificial layer. Once ready for transfer, the sacrificial layer is ablated by a short laser pulse through the carrier substrate to generate gases that propel the component towards a receiving substrate placed in close proximity. What makes LEAP different from LIFT is that it does not rely on the kinetic energy of a plume of vaporized material or solely on the gravitational force to transfer the die. Instead, the UV laser pulse creates a blister in the sacrificial layer, confining the vaporized material within the blister without rupturing it. The force exerted by the blister, in addition to the gravitational force of the die, initiate the contactless die transfer from the wafer onto the receiving substrate where the die is interconnected to the rest of the circuitry.
The blisters resulting from the confined ablation in LEAP act as mechanical actuators allowing for a much better control of die placement and, as a result, more reliable and precise die transfers. This capability, unmatched by the other LIFT-based methods, allowed us to report in 2011 the world’s first successful application of lasers for the fabrication of a functional device (an RFID tag) based on a 50-µm thick, 670-µm square bare die. In 2012 we demonstrated the capabilities of LEAP for embedding RFID chip in paper. Recently, we have reported precision placement of 18-μm thick, 350-μm square dice. To the best of our knowledge, there is no other electronics technology, conventional or nonconventional, capable of high-rate packaging of such ultra-small, ultrathin dice.