Low-cost bump bonding activities at CERN

Abstract

Conventional bumping processes used in the fabrication of hybrid pixel detectors for High Energy Physics (HEP) experiments use electroplating for Under Bump Metallization (UBM) and solder bump deposition. This process is laborious, involves time consuming photolithography and can only be performed using whole wafers. Electroplating has been found to be expensive when used for the low volumes which are typical of HEP experiments. In the low-cost bump bonding development work, electroless deposition technology of UBM is studied as an alternative to the electroplating process in the bump size / pitch window beginning from 20 μm / 50 μm. Electroless UBM deposition used in combination with solder transfer techniques has the potential to significantly lower the cost of wafer bumping without requiring increased wafer volumes.A test vehicle design of sensor and readout chip, having daisy chains and Kelvin bump structures, was created to characterize the flip chip process with electroless UBM. Two batches of test vehicle wafers were manufactured with different bump pad metallization. Batch #1 had AlSi(1%) metallization, which is similar to the one used on sensor wafers, and Batch #2 had AlSi(2%)Cu(1%) metallization, which is very similar to the one used on readout wafers. Electroless UBMs were deposited on both wafer batches. In addition, electroplated Ni UBM and SnPb solder bumps were grown on the test sensor wafers. Test assemblies were made by flip chip bonding the solder-bumped test sensors against the test readout chips with electroless UBMs. Electrical yields and individual joint resistances were measured from assemblies, and the results were compared to a well known reference technique based on electroplated solder bumps structures on both chips. The electroless UBMs deposited on AlSi(2%)Cu(1%) metallization showed excellent electrical yields and small tolerances in individual joint resistance. The results from the UBMs deposited on AlSi(1%) metallization were non-uniform and closer inspection revealed micro cracks at aluminum — electroless nickel interface. UBM deposition was also done for Timepix wafers and solder ball placement process was prototyped with 40 μm balls.