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Laser direct patterning of polymer resins for structured adhesive wafer to wafer bonding

: Zoschke, Kai; Lang, K.-D.

Knechtel, Roy (Ed.) ; Interuniversity Micro-Electronics Center -IMEC-, Louvain:
WaferBond 2017, Conference on Wafer Bonding for Microsystems, 3D- and Wafer Level Integration. Book of Abstracts : 27th - 29th Novmber 2017, Leuven, Belgium
Leuven: IMEC, 2017
Conference on Wafer Bonding for Microsystems, 3D- and Wafer Level Integration (WaferBond) <2017, Leuven>
Fraunhofer IZM ()

In order to overcome the current limitations for structuring of non-cured and non-photo sensitive polymers a new direct pattering method which utilizes laser ablation was evaluated in this work. The approach allows direct mask defined pattering of spin coated and soft baked polymer layers using a 248 nm excimer laser stepper. The system features a 2.5x mask projection with a resulting exposure field of 6.5 x 6.5 mm2. The laser provides a repetition rate of 50 Hz so that 50 ablation pulses per second can be generated. With each pulse laser light penetrates through the mask and projection optic into the surface of the adhesive materials coated on the wafer. The light is absorbed rapidly when entering the polymer causing mechanical shocks which break the molecular bonds of the material causing its physical ablation. With each pulse more material is removed vertically until the polymer is ablated through. The thickness that can be removed with one pulse depends on pulse energy and kind of material to be removed. Based on this, each transparent pattern in the mask can be ablated with 2.5x smaller lateral dimensions into the polymer layer. After the patterning process is done the structured polymer layers can be directly used for wafer to wafer bonding processes. The process concept was developed using CYCLOTENE® 3000 Resin which was well known as permanent polymer for non-patterned wafer to wafer bonding applications so far. The material was spin coated and soft baked at 200 mm wafers to give 5 µm layer thickness. Subsequently, bond frames were structured using the direct laser ablation pattering. In total 24660 bond frames were patterned on each wafer. The wafers were subsequently bonded at 80 °C and 0.2 MPa for 5 minutes with 200 mm glass wafers. Following, the bonded wafer stacks were post bond batch cured at 190°C. Wafer dicing and shear tests of the bonded structures revealed excellent mechanical robustness of the laser structured BCB bond frames. As unique feature compared to other patterning methods laser direct ablation enables structuring of adhesive double layers which can have different cure conditions. Based on this, bond features with different viscosity zones over z height become possible. Bond frames with lower cured polymer portion on top and higher cured polymer base can be generated for example. The higher cured portion enables extended frame height without structure collapse and the lower cured portion enables good material flow over topography at the same time.