Laser Direct Patterning of Dry Etch BCB Adhesive Layers for Low Temperature Permanent Wafer-to-Wafer Bonding

Abstract

To enable advanced wafer level packaging approaches for devices like MEMS, image sensors or optical elements, wafer-to-wafer bonding processes using structured low temperature curable adhesives are required. A lot of Benzocyclobutene (BCB)-based wafer bonding works have been reported in the past showing a broad range of applications and good performance, but also some limitations such as long bond cycles and high cure temperature of 250 °C. In 2013 new process concepts were demonstrated [1], showing that wafer bond cycle time can be reduced to less than 10 min and a post bond batch cure at temperatures below 200 °C can be used to significantly shrink the overall cost of a BCB-based adhesive wafer bonding process. In order to create a patterned BCB bond layer, photo structuring of CYCLOTENE ® 4000 Resin is one solution. However, due to the decreased flow capability of that material after exposure, high bond forces and extended bonding times during wafer bonding as well as nearly flat surfaces with low topography are required for void-free bonding. To overcome these limitations, an increased material flow capability during wafer bonding is required. In this context non-photo sensitive CYCLOTENE ® 3000 Resin is suitable, since it has excellent flow capability in non-cured state. However, non-cured CYCLOTENE ® 3000 Resin cannot be structured with standard dry etching processes using a photo resist layer as mask. In order to enable patterned adhesive bonding based on CYCLOTENE ® 3000 Resin, alternative structuring methods have to be evaluated. One method was presented in [1] which is transfer printing of CYCLOTENE ® 3000 Resin from a help wafer to topography features of the device wafer. Although very good results were obtained, the method is restricted to applications with significant topography to enable the transfer printing. In this work we focus on a new structuring method for non-cured BCB layers formed from CYCLOTENE ® 3000 Resin. The layers were spin coated, baked and subsequently patterned using a 248 nm excimer laser stepper. The system features a 2.5× mask projection with a resulting exposure field of 6.5 × 6.5 mm2 and allows a direct ablation patterning of polymers. By using this method bond frame structures were patterned into 5 μm thick BCB layers at 200 mm silicon wafers. The wafers with the structured adhesive were bonded at 80 °C and 0.2 MPa for 5 minutes with 200 mm glass wafers. The bonded wafer stacks were subsequently post bond batch cured at 190 °C. Wafer dicing and shear tests of the bonded structures revealed excellent mechanical robustness of the BCB bond frames. The paper will review the new BCB wafer bond processes for supporting short cycle times with special focus on the new patterning approach by laser ablation. Process flow description as well as systematical analysis of pattern reproducibility of the new structuring method is part of the discussion.