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.

Fluxless flip-chip bonding using a lead-free solder bumping technique

“…The bumping of test assemblies using the laserjetting process (PacTech SB 2 -jet) for 40-µm solder spheres was successfully demonstrated at CERN. They also found the electroless UBM technology to be promising and cost effective for applications in high-energy physics [5].…”

“…The cost issues in the photolithographic techniques motivated researchers to investigate alternatives. They studied the new controlled-collapse chip connection process (C4NP) [4], gang-ball placement (GBP) [5], Au-stud [6] and laser-jetting technique [7]. The C4NP and GBP are mass transfer techniques, while Au-stud bumping and laser jetting are mask-less sequential processes placing each bump individually.…”

“…The automated bond tool transports the picked balls to the targeted pads with flux assistance. The process has been well adapted for solder-ball diameters from 60 µm to 760 µm [5]. However, the mask requirements in both wafer-level processes, C4NP and GBP, adds to the cost.…”

Pixelized M-π-n CdTe detector coupled to Medipix2 readout chip

“…to reduce the effects of polarization that is common in the M-π-n CdTe detector, 5. to be able to fabricate double sided M-π-n CdTe detector and 6. to obtain edgeless detector behavior in the CdTe detectors for realization of four-side buttable detector modules [1].…”

“…to obtain rectifying contact and lower leakage current levels than with the Schottky contacts, 2. to improve the spatial resolution by reading out the electrons instead of holes, 3. to improve the energy resolution by using the cathode-side as an entrance side for the radiation and read out of electrons, 4. to reduce the effects of polarization that is common in the M-π-n CdTe detector, 5. to be able to fabricate double sided M-π-n CdTe detector and 6. to obtain edgeless detector behavior in the CdTe detectors for realization of four-side buttable detector modules [1].…”