3D flip chip packaging of MEMS sensor

Abstract

Advanced 3D packaging of a Micro Electro Mechanical Systems (MEMS) chip and a CMOS/ASIC Chip was studied. We successfully introduced redistribution process applying two spin coated polybenzoxazole (PBO) polymer layers and two metal layers on 200 mm ASIC wafer. Both MEMS and ASIC bump pad openings were set to 60 µm in diameter. Sputtering and electrochemical plating (ECP) techniques were utilized for metallization. On the Al pads of the sensor Au stud bumps were created. The redistributed ASIC pads were coated with sputtered Au on top of the ECP nickel metal layer and thus Au-Au flip chip bonding was accomplished. The MEMS sensor element in this study was capacitive pressure sensing diaphragm. The diaphragm was made of poly-Si. The pressure range tested was typical barometric range from 35 kPa to 115 kPa. The device operating temperature range from - 40 °C to + 85 °C was tested. Along with the packaging process, solder ball transfer jig was fabricated using bulk silicon wafer. It enabled transfer of eight solder balls to the Chip Scale Packaging (CSP) at one time. The solder ball landing pad was sputtered Au as well. The solder ball pad openings were 300 µm in diameter. Two different size of solder balls were used, 310 µm and 410 µm to ensure enough clearance between CSP and Printed Circuit Board (PCB). Solder balls were consisted of polymer core ball with SnAgCu (SAC) solder metal layers. Several thermo compression bondings were carried out and fine-tune solder ball connections. Functionality was verified by electrical device measurements. To improve productivity, replacement of the Au stud bumps was demonstrated using wafer level ECP to make SnAg µbumps. The plating quality attained within 1 µm height uniformity inside a bonding chip area. SEM observation showed that connection of SnAg micro bump to Au-pad metal was realized.